When Oystercatchers can’t find food

In a rapidly changing world, wintering waders face unprecedented challenges. How much flexibility is there for individuals to cope with issues such as over-fishing of shellfish stocks, habitat removal, pollution, and the effects of rapid climate warming on their food supplies?

Colour-ring records show that many wintering waders tend to be site-faithful, feeding in the same estuaries and even the same small patches year after year. This makes sense if food supplies are reliable and predictable but what happens when there is massive change in food abundance? In a 2021 paper in MEPS, Katharine Bowgen and co-authors describe the impacts of a cockle die-off in the Burry Inlet (part of the Severn Estuary in Wales) on the local population of Eurasian Oystercatchers. Their findings illustrate how important it is to protect networks of sites, rather than individual inlets or estuaries.

Assessing the options

When food supplies are low – or crash suddenly – what can birds such as Oystercatchers do? There are three likely options in these circumstances:

  1. Wait and hope
  2. Move elsewhere and never return
  3. Move elsewhere until conditions improve
When food availability is low, some Oystercatchers may not complete wing moult

If ‘wait and hope’ is the main option then, following an event that reduces food availability or abundance, there should be little evidence of birds dispersing, measures of annual survival might be depressed for a period and, once conditions improve in the area, young birds might be likely to fill the spaces available. This should lead to higher proportions of sub-adults in the period immediately after such a hiatus and a steady increase in numbers.

Movements to other sites, whether temporary or permanent ought to be detectable from mid-winter counts and through reports of ringed birds. The pattern after a permanent shift in birds would be similar to that following mortality; again, young birds might be expected to take advantage of food resources as they recover In reality, of course, individual birds that use a site may opt for any one of the three options, creating a mixed picture.

Oystercatcher numbers

Over recent decades, numbers of Eurasian Oystercatchers have declined. In 2015 the species was reclassified as “Near Threatened” on the IUCN’s Red List (Birdlife International) and “Vulnerable” within Europe. It is also Amber listed on the UK’s Birds of Conservation Concern list, due to its European status, the concentration of its wintering population in protected sites and the international importance of UK breeding and wintering populations.

Colour-ring sightings can improve survival estimates

The changes in Burry Inlet Oystercatcher numbers should be viewed within a pattern of national declines. Since the 1990s, winter numbers on UK estuaries have dropped by a third, back to levels seen in the 1970s. Patterns around Great Britain vary (see below); Welsh numbers on estuaries, as assessed by WeBS, have held up well but there has been a 25-year period of decline in England and there was a sudden fall in Scottish numbers at the start of this century.

The Burry Inlet

The intertidal mudflats of the Burry Inlet in south Wales are of international importance for non-breeding waterbirds of various species, including the Eurasian Oystercatcher. This SPA (Special Protection Area) is part of The Carmarthen Bay and Estuaries SAC (Special Area of Conservation). Burry Inlet received significant conservation attention back in the 1970s, when the UK government gave permission for 10,000 Oystercatchers to be shot, to protect cockle stocks. This decision was taken despite the objections of conservationists in the UK and Norway, the latter being the summer home of many of these birds. The fact that cockle numbers continued to fall after the cull was an embarrassment, suggesting that Oystercatcher predation was not the only factor at play. Fast forward another 25 years, to the start of the period considered in this paper …

The cockle population in the Burry Inlet SPA declined from 1997 to 2004, before an abrupt ‘crash’ in stocks between 2004 and 2010 which was linked to increased mortality in older cockles, which are particularly important to commercial shellfishers. There are suggestions that losses were associated with warmer summers and sewage releases in periods of wet weather. While there has been some recovery since that period, stocks of larger cockles are still very low.

As cockles are a major prey species for Oystercatchers, the loss of larger individuals may place significant pressure on their populations, as has also been seen in the Dutch Wadden Sea and in The Wash SPA in the UK. Studies in the latter area, by BTO scientists and using data from the Wash Wader Research Group, linked declines in Oystercatcher survival and numbers to years of low cockle numbers (Atkinson et al. 2003 and Atkinson et al. 2005).

When trying to understand Oystercatcher responses to cockle changes in the Burry Inlet and the wider Carmarthen Bay SAC, Katharine Bowgen and colleagues had two main data-sets available to them. The Wetland Bird Survey, and the Birds of Estuaries Enquiry that preceded it, have provided fifty years of data on winter numbers for the Burry Inlet and other local and regional sites. The relative importance of different areas within Carmarthen Bay and the Burry Inlet were established using Low Tide Counts (see below).

Ringing operations also contributed data on the movements of marked birds, individual body masses that could be used to assess the condition of trapped birds, retrap information from which survival rates can be estimated, and opportunities to assess how many juveniles and immatures there are within samples of caught birds. November estimates of cockle biomass were available for the period from 1993 to 2008 (NRW/CEFAS).

What happened to Burry Inlet Oystercatchers?

Here are some of the key findings. Please see the paper for methods and full results. The study was funded by CCW (now NRW).

Extracting Oystercatchers from the cannon-net, having made a successful catch
  • Adult Oystercatchers were found to be in better body condition than immatures and juveniles. Non-breeders may spend three or more years in sites such as the Burry Inlet before first returning to breeding areas.
  • There was considerable variation in annual body condition indices. Two features stood out. Condition was lower in 2005, the winter following the crash in cockle stocks, and improved in the following year. A similar bounce-back could be seen after a particularly cold winter (2010).
  • From recapture data, apparent survival post-2000 was positively correlated with total cockle biomass. Apparent adult survival dropped from an average of 99.3% (range 98.3-99.9%) to 78.5% (range 68.5-84.3%) during the years following the crash in cockle stocks (2004), before rising back to 99.5% (range 99.0-99.9%).
  • In a previous BTO report to CCW (Niall Burton, Lucy Wright et al, 2010) the survival impacts of the Burry cockle crash appeared higher. This effect was diluted with the addition of eight extra years of data presented in this paper. Shorter snapshots of data do not fully capture changes in survival rates for long-lived wader species.
  • Cannon-netting Oystercatchers is not easy! The lack of consistent annual catching success and biases associated with, for instance, just catching the edge of a flock (where juveniles tend to be concentrated), are probably reflected in the fact that no recruitment patterns could be established.
  • WeBS Core Count data showed a significant long-term decrease in the population of Oystercatchers wintering in the Burry Inlet and a long-term increase in the population in Carmarthen Bay. During the period 1997 to 2017 there was a correlation of the two sets of figures – as Burry numbers declined, Carmarthen went up and vice versa (see figure below). Counts of Oystercatcher in the Burry Inlet were weakly associated with cockle biomass in the estuary. Carmarthen Bay counts were more strongly linked to Burry Inlet cockle biomass, increasing as cockle supplies dropped in the Inlet.

Take-home messages

In the study by Katharine Bowgen and colleagues, an apparently underexploited area within the Carmarthen Bay SAC became a vital resource when food supplies collapsed in the Oystercatchers’ preferred feeding area.

Understanding how birds can (and may need to) respond to changing food resources is important, given ongoing pressures from shell-fishers and the fact that the distribution of invertebrate prey stocks may be affected by climate change. The ongoing cockle decline in the Burry Inlet is of concern to both the fishermen, reliant on the stocks, and to conservation managers monitoring bird populations. This study suggests that Oystercatchers may be able to adapt during periods of stress but only if alternative foraging areas are available in the local vicinity.

The analysis of long-term datasets allows more accurate understanding of incidents such as the cockle crash investigated here and improves our abilities to manage their effects on longer-lived species such as waders. Only through long-term monitoring is it possible to fully understand the consequences of major changes in species’ resources and how individuals might adapt to cope with their impacts.

Population level effects

The following three WaderTales blogs contain information about how wintering conditions in particular study areas can affect wader populations.

Sanderling migration

In A place to roost, there is a section about the consequences for local Redshank when Cardiff Bay was permanently flooded, with links to three important papers. Birds that moved elsewhere had difficulty in maintaining their body condition in the winter following removal of their feeding habitat and continued to exhibit lowered survival rates in subsequent years.

In Travel advice for Sanderling,there is clear evidence that poor wintering conditions affect survival rates, the probability of breeding in the first summer and the timing of spring arrival in Greenland. All of these three factors can have population-level effects for the species.

In Gap year for sandpipers, we learn that Semipalmated Sandpipers may not breed every year, depending upon the condition they are in when it is time to migrate.

These three stories are all relevant to the Burry situation. Oystercatchers that were in poor condition at the end of a winter, either because they stayed in the Inlet and had fewer resources or because they moved to new sites, may not have had the resources to migrate to their breeding areas in some years or could have migrated later, either of which may reduce the number of potential breeding attempts within a season (see Time to nest again?).

Conservation implications

Although the analyses presented here were undertaken in order to understand what happened in the Burry Inlet, a site designated in part because of the high Oystercatcher counts, the authors emphasise just how important networks of sites are to wider shorebird conservation issues, especially if there is a rapid change in the quality of a core area. It is not sufficient just to protect the very best sites.

This paper could not have been written without the work of volunteer counters and ringers

With coastal wader populations exhibiting long-term declines globally, understanding how they respond to changes in their prey is important, especially given the potential for warming seas to affect invertebrate populations. In this context, the Burry Inlet study demonstrates the value of long-term WeBS counts and the efforts of local ringers. The contribution of volunteers is warmly acknowledged at the end of the paper.

Resilient protected area network enables species adaptation that mitigates the impact of a crash in food supply. Bowgen, K.M., Wright, L.J., Calbrade, N.A., Coker, D., Dodd, S.G., Hainsworth, I., Howells, R.J., Hughes, D.S., Jenks, P., Murphy, M.D., Sanderson, W.G., Taylor, R.C. and Burton, N.H.K. MEPS. DOI:https://doi.org/10.3354/meps13922


WaderTales blogs are written by Graham Appleton (@GrahamFAppleton) to celebrate waders and wader research. Many of the articles are based on published papers, with the aim of making shorebird science available to a broader audience.

WaderTales blogs in 2021

The global reach of WaderTales

There were 17 new WaderTales blogs in 2021. Although many have a UK focus, the map below shows the international reach of the series. The next few paragraphs summarise some of the exciting new stories that were covered during the year, using technologies as diverse as aerial surveys, satellite tracking and genetics. Many of the papers could not have been written without huge input from shorebird counters and colour-ring readers.

Amazing migration

Many WaderTales blogs focus on migration, with more and more stories appearing as geolocators and satellite tags allow researchers to follow the whole migratory cycle. Three 2021 blogs illustrate how much there is still to learn.

Flying high with Great Snipe reveals that these amazing migrants, travelling from Sweden to the Congo basin, can achieve heights that would allow them to almost clear Everest (despite there being no need to fly this high). The paper upon which the blog is based focuses on differing altitudes of daytime and nocturnal flights. Why does a Great Snipe fly twice as high during daylight hours?

Winter conditions for Whimbrel asks whether conditions experienced in Africa affect breeding performance in Iceland. Now that researchers can study individual birds in the breeding season and wintering areas, it is becoming possible to look more closely at carry-over effects. At the end of the blog, there’s what is hopefully a helpful summary of all of the research relating to Icelandic Whimbrel.

The flock now departing reveals fascinating details about Curlew migration, with descriptions of four occasions when two tagged birds ended up in the same migratory flock. With more birds being satellite-tagged, it was only going to be a matter of time until researchers picked up cases of birds travelling together. So much food for thought in the paper at the heart of this blog!

Conservation and new technologies

There are huge conservation concerns with regard to many of the world’s waders. The next four blogs all illustrate how technological advances can help us to understand issues that are threatening shorebirds. Three were published in 2021 and the fourth includes a couple of updates to a blog from 2020.

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Grassland management for Stone-curlew explains how GPS tags are revealing the preferred feeding habitats for Stone-curlews that breed in Breckland (eastern England). The creation of cultivated plots within extensive areas of grassland can provide Stone-curlews with good nesting habitat but do birds feed in these areas too? A small-scale study provided answers and showed that nesting birds are prepared to fly a long way to find food – especially to forage in pig fields.

Remote monitoring of wader habitats. Radar can be used to assess the suitability of large areas of grassland for breeding waders. The focus of the paper is upon the performance of an agri-environment scheme in Estonia, where EU funding is being directed towards breeding wader species, particularly Dunlin, Ruff, Black-tailed Godwit and Redshank. The results are of broader relevance to conservation biologists who are seeking to monitor vegetation growth and the encroachment of shrubs and trees, especially associated with a warming climate, afforestation and farmland abandonment.

Following Sociable Lapwings reveals the migration routes used by this globally-threatened species. This is vital information for conservationists who are concerned about excessive hunting of adults. Now that they know the key hotspots that are important during spring and autumn migration, as well as the areas in which birds spend the winter, it is hoped that illegal and unsustainable hunting might be reduced. Current average annual survival levels of adults are way too low. Satellite studies have revealed a previously-unknown (though suspected) third migration route.

Spoon-billed Sandpiper: track and trace was published in 2020 but has been extended to include summaries of 2021 papers, with improved population estimates (not good news) and information as to how local tracking information may be used to assess changes in habitat quality.

A focus on subspecies

Conservation priorities often relate to geographically-referenced populations or subspecies, rather than whole species. For instance, islandica Black-tailed Godwits numbers are increasing but limosa, the heartland of which is the Netherlands, is a subspecies causing huge concern. Three WaderTales blogs from 2021 focus on subspecies.

New Bar-tailed Godwit Subspecies explains why taymyrensis should be considered as two separate populations or subspecies, one of which travels from western Siberia to the Arabian Gulf. The two proposed subspecies have different migration routes and operate on different annual calendars. The newly-named yamalensis subspecies is subject to very different conservation challenges, especially in areas around the Caspian Sea and Aral Sea.

Subspecies, connectivity and conservation in shorebirds concerns the Red Knot that spend the non-breeding series in Chile, and asks whether the concept of a subspecies may distract from local conservation priorities.

Dunlin: tales from the Baltic focuses upon declines of a disappearing population of schinzii Dunlin but set in a global context. The focal paper examines long-term effects of carrying a geolocator but the potential negative effects of tracking need to be viewed in the context of a population that has dropped by 80% in 40 years. Colour-ring studies show that annual apparent survival rates of Baltic birds dropped from 0.817 to 0.650 between 1990 and 2006, equivalent to a doubling of the chance of dying in any given year.

Birds on beaches

Three of the blogs in 2021 relate to waders/shorebirds that are found on beaches

In amongst the tidewrack promotes new research into the way that roosting and resting waders make best use of microhabitats created by fresh and older beds of seaweed. The main species using this South Australian study site are Red-necked Stint and Double-banded Plover but Bar-tailed Godwit, Curlew Sandpiper, Ruddy Turnstone and Sanderling contributed data too. There’s a neat experimental set-up involving plastic golf balls!

Waders on the coast reports on the waders that winter around the open coasts of the United Kingdom, emphasising the importance of these unprotected habitats and detailing changes in numbers over 18 years. It is estimated that there are 250,000 waders on the UK’s coast but numbers of most species are lower than they were.

On the beach: breeding shorebirds and visiting tourists discusses how much space is being taken away from breeding Ringed Plovers and Oystercatchers, in Norfolk and Suffolk (UK). One of the neat features of the research at the heart of this blog is a series of aerial counts of tourists.

Oystercatchers

Two Oystercatcher papers are featured in 2021 WaderTales blogs, relating to two species with very different migratory behaviour. African Oystercatchers on Robben Island are resident, spending the time there either breeding or moulting. Most of Iceland’s Eurasian Oystercatchers migrate south for the winter, as discussed in the 2020 blog Which Icelandic Oystercatchers cross the Atlantic?

Who eats African Oystercatcher eggs? There is a complex interplay of potential predators on South Africa’s Robben Island. African Oystercatchers nesting near the Kelp Gull colony do much better than pairs that nest around the rest of the island. The gulls help to dissuade patrolling Mole Snakes, which are now the most significant predators of birds’ eggs.

Oystercatcher migration: the Dad Effect reveals that Icelandic chicks adopt the same migratory behaviour as their father (not their mother). Youngsters that cross the Atlantic migrate much later than their fathers but something about the way they are brought up must affect the ‘decision’ as to whether to migrate or not.

Conservation in the UK

Returning home, there are three blogs about the UK’s red-listed waders

England’s Black-tailed Godwits diagnoses the reasons for the recent decline in breeding numbers, following a period of rapid increase that ended in 2006. The positive message in this blog is that, if the breeding conditions are right, it is possible for the number of breeding pairs to nearly double in ten years.

More Curlew chicks needed has at its core a paper about survival rates of breeding and wintering Curlew. Even in a period of historically low annual adult mortality, UK breeding numbers have continued to fall. Based on annual survival rates of adults and current productivity, it would appear that British Curlews need to successfully fledge an extra 10,000 chicks every year – just to stop the decline in numbers.

Eleven waders on UK Red List is an update of Nine red-listed UK waders. Dunlin and Purple Sandpiper were added to the red list of Birds of Conservation Concern in 2021. The blog explains how each species has earned its place on the list.

Blogs from previous years

WaderTales blogs in 2020

WaderTales blogs in 2019

WaderTales blogs in 2018

WaderTales blogs in 2017

WaderTales blogs are written by Graham Appleton, to celebrate waders and wader research. Many of the articles are based on previously published papers, with the aim of making wader science available to a broader audience.The intention is to add one or two new blogs each month. You can sign up to receive an e-mail notification when a new one is published. There’s a full list of blogs here.

Eleven waders on UK Red List

If you ask British birdwatchers to name the eleven wader species that are causing the most conservation concern in the UK, they would probably not include the Ringed Plover. Curlew may well be top of the list, even though we still have 58,500 breeding pairs in the UK*, but would people remember to include Ruff?

This blog was originally written to coincide with the publication of Red67, an amazing collaboration by artists and essayists that highlighted and celebrated the 67 species on the UK red list, nine of which were waders. Following the publication of the fifth edition of Birds of Conservation Concern on 1 Dec 2021, this blog has been revised, to include Dunlin and Purple Sandpiper. The UK Red List now contains 70 species.

*Avian Population Estimates Panel report (APEP4) published in British Birds

What’s a Red List?

The UK Red List is made up of a strange mixture of common and rare species. Nobody will be surprised to see fast-disappearing Cuckoo, Turtle Dove and Willow Tit, but why are 5.3 million pairs of House Sparrow in the same company? The list is very important because it helps to set the agenda for conservation action, the way that money for research is distributed and focuses attention during planning decisions. The main criteria for inclusion are population size – hence the inclusion of species that are just hanging on in the UK, such as Golden Oriole – and the speed of decline of common species. Data collected by volunteers, working under the auspices of the British Trust for Ornithology, measured a population decline for House Sparrow of 70% between 1977 and 2017, which is worrying enough to earn this third most numerous breeding species in the UK a place in Red67.

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In his foreword to Red67, Mark Eaton, Principal Conservation Scientist for RSPB, explains how listing works. The Birds of Conservation Concern (BOCC) system, through which the Red List and Amber List are determined, uses a strict set of quantitative criteria to examine the status of all of the UK’s ‘regularly’ occurring species (scarce migrants and vagrants aren’t considered), and uses a simple traffic light system to classify them. There are ‘Red’ criteria with thresholds for rates of decline in numbers and range, historical decline and international threat (if a species is considered globally threatened it is automatically Red-listed in the UK), together with a range of other considerations such as rarity, international importance of UK populations, and how localised a species is. If a species meets any of the Red List criteria it goes onto the Red List.

The Red67 book – words meet art

Red67 was the brainchild of Kit Jewitt, a.k.a. @YOLOBirder on Twitter. It’s a book featuring the 67 Red-listed birds of Birds of Conservation Concern 4, each illustrated by a different artist alongside a personal story from a diverse collection of writers. Proceeds supported Red-listed species conservation projects run by BTO and RSPB. Kit describes Red67 as 67 love letters to our most vulnerable species, each beautifully illustrated by some of the best wildlife artists around, showcasing a range of styles as varied as the birds in these pages. My hope is that the book will bring the Red List to a wider audience whilst raising funds for the charities working to help the birds most at need.

This blog is about the nine waders in the book and the two additions to the list (Dunlin and Purple Sandpiper), but there are 58 other fascinating species accounts and wonderful artworks. Each species account starts with a quote from the story in the book and is accompanied by a low-resolution version of the artwork (Ringed Plover is illustrated above).

Lapwing

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“It’s the crest that does it for me – that flicked nib stroke, the artist’s afterthought” – Lev Parikian

The Lapwing used to nest across the whole of the United Kingdom and was a common bird in almost every village. It’s still the most numerous breeding wader in the UK, with 97,500 pairs (APEP4), beating Oystercatcher by just 2,000 pairs. Numbers dropped by 54% between 1967 and 2017, according to BirdTrends 2019, published by BTO & JNCC. Huge losses had already occurred over the previous two centuries, as land was drained and vast numbers of eggs were collected for the table. The Lapwing is now a bird associated with lowland wet grasslands and the uplands, rather than general farmland.

Red-listing has been important for Lapwing, increasing the profile of the species and encouraging the development of specific agri-environment schemes targeted at species recovery. These include ‘Lapwing plots’ in arable fields and funding to raise the summer water tables in lowland grassland. Several WaderTales blogs describe efforts to try to increase the number of breeding waders in wet grassland, especially Toolkit for Wader Conservation. The loss of waders, and Lapwings in particular, from general farmland is exemplified in 25 years of wader declines.

Ringed Plover

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“They gather at high tide like shoppers waiting for a bus: all facing the same direction, and all staring into the distance” – Stephen Moss

One of the criteria that the BOCC panel takes into account, when constructing the Red List, is the responsibility the UK has for a species or subspecies in the breeding season, during winter or both. The Ringed Plovers we see in the UK in the winter are almost exclusively of the hiaticula subspecies; birds that breed in southern Scandinavia, around the Baltic, in western Europe and in the UK. There are only estimated to be 73,000 individuals in this subspecies, so the 42,500 that winter in the UK constitutes a large percentage of the Ringed Plovers that breed in many of these countries.

The Wetland Bird Survey graph alongside shows a decline of over 50% between 1989 and 2014. At the start of the period, Ringed Plover numbers were at an all-time high but this is still a dramatic and consistent drop. Numbers have stabilised and may even have increased slightly but Ringed Plovers need some good breeding years. Disturbance is an issue for breeding Ringed Plovers, which share their beaches with visitors and dogs, and could also potentially be a problem in the winter (see Disturbed Turnstones).

Dotterel

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“I want you in the mountains. Summer breeze. At home. Doing your thing. So don’t go disappearing on us, okay?” – Fyfe Dangerfield

The Dotterel is a much clearer candidate for Red67 – there’s a small population in a restricted area and numbers have fallen. The detailed reasons for decline may still need to be nailed down but candidate causes such as declining insect food supplies and the increasing numbers of generalist predators are probably all linked to a changing climate – squeezing Dotterel into a smaller area of the mountain plateaux of Scotland.

There’s a blog about the decline in Dotterel numbers called UK Dotterel numbers have fallen by 57%, based upon a paper that uses data up until 2011. At this point, the population was estimated at between 280 and 645 pairs. There has been no suggestion of improvement since that blog was written. Interestingly, Dotterel may have a way out of their predicament, as we know that marked individuals move between Scotland and Norway in the same breeding season. See also Scotland’s Dotterel: still hanging on.

Whimbrel

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“How often do Whimbrels pass overhead nowadays? Unseen and unheard, their calls mean nothing to most of us” – Patrick Barkham

Most British and Irish birdwatchers think of Whimbrel as spring migrants, enjoying seeing flocks of Icelandic birds when they pause on their way north from West Africa (see Iceland to Africa non-stop). There is a small, vulnerable population nesting almost exclusively on Shetland. The latest estimate is 310 pairs (2009), down from an estimate of 530 pairs, published in 1997. Many pairs have been lost from Unst and Fetlar and this blog about habitat requirements might give clues as to why: Establishing breeding requirements of Whimbrel.

The curlew family is in trouble across the Globe, potentially because these big birds need so much space (see Why are we losing our large waders?)

Curlew

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“… achingly vulnerable in a world that is battling to hold onto loveliness” – Mary Colwell

What more can be said about Curlew, ‘promoted’ to the red list in 2015 and designated as ‘near threatened’ globally. Most significant is the story from Ireland, where 94% of breeding birds have disappeared in just 30 years. These blogs provide more information about the decline and review some of the reasons.

There are more Curlew-focused blogs in the WaderTales catalogue.

Black-tailed Godwit

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“A glimpse of terracotta is obscured by ripples of grass, dipping gently in the breeze” – Hannah Ward

Winter Black-tailed Godwit numbers are booming but these are islandica – birds that have benefited from warmer spring and summer conditions in Iceland, as you can read here in: From local warming to range expansion. Their limosa cousins are in trouble in their Dutch heartlands (with declines of 75%) and there have been similar pressures on the tiny remaining breeding populations in the Ouse and Nene Washes. Here, a head-starting project is boosting the number of chicks; so much so that released birds now make up a quarter of this fragile population. Red-listing has shone a spotlight on this threatened subspecies, attracting the funding needed for intensive conservation action.

Ruff

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“They look a bit inelegant – a small head for a decently sized bird, a halting gait, and that oddly vacant face” – Andy Clements

There are two ways for a species to be removed from the Red List – extirpation (extinction in the UK) and improvement. Temminck’s Stint came off the list in 2015, having not been proven to breed since 1993, and Dunlin was moved to Amber at the same time. Ruff are closer to extirpation than they are to the Amber list. There is a spring passage, mostly of birds migrating from Africa to Scandinavia and the Baltic countries, and some males in glorious breeding attire will display in leks.

250 years ago, Ruff were breeding between Northumberland and Essex, before our ancestors learnt how to drain wetlands and define a hard border between the North Sea and farmland. Hat-makers, taxidermists and egg-collectors added to the species’ woes and, by 1900, breeding had ceased. The 1960s saw a recolonisation and breeding Ruff are still hanging on. There are lekking males causing excitement in sites as disparate as Lancashire, Cambridgeshire and Orkney, and there are occasional nesting attempts. Habitat developments designed to help other wader species may support Ruff but the situation in The Netherlands does not suggest much of a future. Here, a once-common breeding species has declined to an estimated population of 15 to 30 pairs (Meadow birds in The Netherlands).

Red-necked Phalarope

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“… snatching flies from the water in fast, jerky movements, droplets dripping from its slender beak” – Rob Yarham

Red-necked Phalaropes that breed in Shetland and a few other parts of northern Scotland appear to be an overflow from the Icelandic population; birds which migrate southwest to North America and on to the Pacific coastal waters of South America. This BOU blog describes the first track revealed using a geolocator.

The Red-necked Phalarope was never a common breeder and came under pressure from egg-collectors in the 19th Century. Numbers are thought to have recovered to reach about 100 pairs in Britain & Ireland by 1920. Numbers then fell to about 20 pairs by 1990, so the latest estimate of 64 pairs (The Rare Breeding Birds Panel) reflects conservation success. Given the restricted breeding range and historical declines, it is unlikely that the next review will change the conservation status from Red to Amber, despite the recovery of numbers.

Woodcock

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“… taking the earth’s temperature with the precision of a slow, sewing-machine needle” – Nicola Chester

The presence of Woodcock on the Red List causes heated debate; how can this still be a game species? Red-listing is indisputable; the latest survey by BTO & GWCT showed that there was a decline in roding males from 78,000 in 2003 to 55,000 in 2013, with the species being lost from yet more areas of the UK. Each autumn, the number of Woodcock in the UK rises massively, with an influx of up to 1.4 million birds. Annual numbers depend upon seasonal productivity and conditions on the other side of the North Sea. A recent report on breeding wader numbers in Norway, Sweden and Finland, shows that breeding populations of Woodcock in this area are not declining (Fennoscandian wader factory).

The UK’s breeding Woodcock population is under severe threat from things such as increased deer browsing and drier ground conditions but winter numbers appear to be stable. The difference in conservation status between breeding and wintering populations is reflected in the fact that Woodcock is on both the Red List and the Quarry List, for now. There is a WaderTales blog (Conserving British-breeding Woodcock) that discusses ways to minimizes hunting effects on British birds. These guidelines from GWCT emphasise the importance of reducing current pressures on British birds.

Dunlin (added to Red List 2021)

There are good reasons for the addition of Dunlin to the UK Red List; breeding numbers of the schinzii race have declined significantly and the number of wintering alpina Dunlin in the UK dropped 10,000 between two periods that were only eight years apart.

The fall in breeding numbers is probably the bigger problem, as indicated by range reductions in successive breeding atlas periods (1968-72, 1988-91 and 2008-11). It’s hard to monitor what is happening to numbers on a year-by-year basis because the breeding distribution is too patchy and thin to be picked up via the Breeding Bird Survey. Alarm bells are being raised in Ireland, where numbers have dropped to between 20 and 50 pairs (Status of Rare Breeding Birds across the island of Ireland, 2013-2018), and in the Baltic, where numbers have fallen by 80% since the 1980s. There’s a WaderTales blog about Finnish Dunlin.

The winter decline in the UK may or not be an issue, although it has to be considered as part of large-scale moderate losses across the whole of western Europe. There is a theory that, over the years, as winter conditions on the continental side of the North Sea have become less harsh, new generations of juvenile alpina have settled in countries such as the Netherlands, instead of continuing their southwesterly migrations from northern Russia. Once a wintering site has been chosen, individuals are site faithful, so one ends up with newer generations on the continent and ageing adults in the UK and Ireland. WeBS counts for Dunlin are half what they were 25 years ago.

Purple Sandpiper (added to Red List 2021)

The rocky coasts of the UK are home to Purple Sandpipers from the Arctic, with a suggestion that North Sea coasts, south of Aberdeen, mainly play host to birds from Spitsbergen and northern Scandinavia, with Greenlandic and Canadian birds more likely to be found further north and on the Atlantic coast. Coastal numbers declined by 19% between 1997/98 and 2015/16, according to the Non-estuarine Waterbird Survey, and WeBS counts suggest numbers have at least halved over a period of 25 years. The Highland Ringing Group has shown that the number of young Purple Sandpipers has been declining on the Moray Firth, suggesting either a period of relatively poor breeding success for birds migrating from the northwest or short-stopping by new generations of youngsters, as discussed for Dunlin. Perhaps they are wintering in Iceland these days?

In conclusion

blog book

The Red List creates some strange bedfellows. In the book, Turtle Dove follows Herring Gull; a bird with links to love and romance and another with at best the charm of a roguish pirate. But the List works; it creates an evidence-base that help those who devise agricultural subsidy systems, advise on planning applications, license bird control and prioritise conservation initiatives.

Red67 sought to raise awareness of the UK’s most at-risk bird species, nine of which were waders, and to raise money for BTO and RSPB scientists to carry out important research. It’s a lovely book that captures the thoughts and images of a generation of writers and artists. You can learn more about the project, order the book and buy some Red Sixty Seven products by clicking here.

Perhaps a second edition, called Red Seventy, might be produced, to reflect the changing fortunes of the UK’s birds? Worryingly, the first Red List, produced in 1990, only had 36 species on it.


GFA in Iceland

Graham (@grahamfappleton) has studied waders for over 40 years and is currently involved in wader research in the UK and in Iceland.  He was Director of Communications at The British Trust for Ornithology until 2013 and is now a freelance writer and broadcaster.

Who eats African Oystercatcher eggs?

As ornithologists, focused as we are upon birds, we perhaps find it hard to fully acknowledge birds’ eggs as key ‘consumables’ in the food web – a ready source of protein for everything from snakes to deer.

Nest cameras have shown that clutches of ground-nesting waders are taken opportunistically, by grazing sheep for instance, but there are also some species – or individuals – that are specialist egg hunters. See Prickly problems for breeding waders (hedgehogs) and Curlews and foxes in East Anglia (sheep).

In their paper in Wader Study, Itxaso Quintana, Rio Button & Les Underhill describe a single-year study of the predation of African Oystercatcher nests on Robben Island, best known for the prison where Nelson Mandela was held for eighteen years, and for its important seabird colonies. Introductions and eradications on Robben Island have created a far-from-natural food-web that is ever-changing. The situation that Itxaso, Rio and Les found in the 2019/20 breeding season included Kelp Gulls ‘protecting’ African Oystercatcher nests from the attentions of Mole Snakes – something that makes little sense unless set in a historical context, as the authors do in their paper.

What a mess!

Homo sapiens has interfered spectacularly on Robben Island, in the same way as in so many other places, with exploitation of seals, the introduction of an array of large mammal species from the mainland during the apartheid era, and the introduction of rabbits, fallow deer and cats. By 2019/20, the rabbits and almost all of the deer had been removed, encouraging the regeneration of ground cover, and over 90% of the cats had been culled. The remaining threats for African Oystercatcher nests were expected to come mostly from Kelp Gulls, native Mole Snakes and the small number of remaining cats.

Checking out an Oystercatcher’s nest on the shoreline

The 550 African Oystercatchers on Robben Island account for 8% of the species’ population, making this a very important breeding site. This total comprises both nesting pairs and non-breeding birds. African Oystercatchers do not migrate, relying on local shellfish supplies year-round. In 2000, the species was classified as ‘near threatened’ but, thanks to the spread of the invasive Mediterranean Mussel along the South African coast, numbers have recovered. Native shellfish might be in trouble but at least the African Oystercatcher can now be considered to be of ‘least concern’!

The 2019-20 breeding season

Robben island has a coastline which is less than 10 km in length, enabling the authors to monitor 158 nesting attempts of what is thought to have been 133 pairs of African Oystercatchers. A further 300 non-breeding oystercatchers were also present. There were three main study sections, corresponding to the north end of the island (63 nests), the south end of the island (64 nests) and the east side (29 nests). The nest success rates of nests in the north and south were very different, with rates in the east being intermediate.

Below the gull colony in the north of the island, 45 out of 63 African Oystercatcher nests were successful (71%) with one failure associated with Mole Snakes.

Away from the gull colony, in the south of the island, only 14 out of 64 African Oystercatcher nests were successful (22%) with 17 failures associated with Mole Snakes and the reason for most failures unknown.

Breeding Kelp Gulls are spreading across the northern part of Robben Island

There was far more evidence of Mole Snake activity in the south of the island, as can be seen in the map, with individual snakes seemingly ‘patrolling the shoreline just above the spring high tide level, where African Oystercatchers lay their eggs’. In the north, where African Oystercatchers nest on the shoreline immediately below the Kelp Gull colony, snakes were much less conspicuous. You can read more about this in the paper.

Two decades of research

The long-term study of African Oystercatchers on Robben Island started in 2001. Since then, the population has increased fourfold and the number of nests has almost doubled. There have been many changes over this period, as you can read in the paper, but the most significant one for African Oystercatchers is the arrival of Mediterranean Mussels, first noticed in South African waters in 1979 and already colonising the shoreline of Robben Island by 2003. These invasive mussels provide more food than native shellfish and the authors suggest that this has fed through into higher oystercatcher nesting densities.

The beak of an African Penguin is enough to deter Mole Snakes

Over the two decades, there has been a massive change in the populations of potential predators. In the early part of the research period, Mole Snakes were considered relatively unimportant, in terms of predation pressure on African Oystercatchers. Introduced cats, however, then became a major problem, with numbers growing between 2001 and 2005, to such an extent that at least 83% of African Oystercatcher nests were predated in the 2004/05 breeding season. Culls in 2005 and 2006, followed by continued controls, have lowered cat numbers to fewer than ten individuals.

There were no Kelp Gulls breeding on Robben Island until 2000/01, when the first five nests were found. More and more birds now make the short commute to Cape Town, to scavenge, and 2829 gull nests were recorded in 2019/20. Like other large Larus gulls, Kelp Gulls have a reputation as egg thieves. However, on Robben Island, where there are few people disturbing nesting African Oystercatchers, and forcing them to leave their nests, the gulls seem to cause few problems. Instead of being a threat, Kelp Gulls attack Mole Snakes, thereby protecting the eggs of the African Oystercatchers.

For Kelp Gulls, it’s only a short commute to Cape Town, where they scavenge for food

Four other species have benefited from reduced cat numbers. As hoped, numbers of Hartlaub’s Gulls and Swift Terns have both increased, while Mole Snakes have experienced higher survival because the eggs of these two species are available in the austral autumn and winter. African Penguins have benefited too, as a consequence of reduced cat predation, and their sharp beaks can deal with Mole Snakes.

What next?

Hatched Oystercatcher chick and another one on the way

The current situation seems to suit all of the species considered here. Removing introduced herbivores has provided more suitable habitat for snakes, and nesting numbers of key seabirds and African Oystercatchers have all increased since most cats were culled.

Robben Island is not a natural ecological system, however. Major perturbations have happened over decades, with introductions, extirpations, culls and the arrival of invasive species. For the moment. the authors see no reason for Kelp Gulls and/or Mole Snakes to be controlled, in order to support African Oystercatcher numbers or protect important seabird populations.

Cold searching the area above the tide-line to locate nests of African Oystercatchers (Mole Snakes do the same)

One thing that seems almost certain is that the situation will change again: the local Mediterranean Mussel population could collapse, for example due to disease; a pollution incident in the busy sea lanes into the port of Cape Town could suddenly impact birds and/or their food supplies; the grinding effects of climate change, particularly the risk of increased storminess, could slowly upset the equilibrium; diseases such as avian influenza, currently affecting Cape Cormorants, could spread further; or the cat population could explode again. Robben Island looks like a fascinating place in which to study complicated predator-prey interactions; here’s hoping that long-term monitoring will continue.

Paper

The full paper is available in Wader Study, the journal of the International Wader Study Group.

African Oystercatchers on Robben Island, South Africa: The 2019/2020 breeding season in its two decadal context.
Itxaso Quintana, Rio Button & Les G. Underhill. Wader Study.


WaderTales blogs are written by Graham Appleton (@GrahamFAppleton) to celebrate waders and wader research. Many of the articles are based on published papers, with the aim of making shorebird science available to a broader audience.

New Bar-tailed Godwit Subspecies

The description of a new subspecies of Bar-tailed Godwit begs two questions, ‘How do we identify new subspecies?’ and ‘Is the concept of a subspecies helpful?’. Put simply, the answers are ‘That depends’ and ‘Yes, especially when subspecies are the focus for conservation action’.

A new study of Bar-tailed Godwits, and the proposal to identify Limosa lapponica yamalensis as a new race, usefully highlight the importance of the flyway between Siberia and the Arabian Sea and the challenges being faced by waders that use this migration route.

Limosa lapponica yamalensis

Bar-tailed Godwits breed across the whole of northern Eurasia and in Alaska. Since the last Ice Age, following glacial retreat, new breeding opportunities in northern latitudes have become available to waders. The emergence of separate migration flyways, linking wintering and breeding areas, has led to a divergence of the species into a number of distinct populations, some of which have already been defined as subspecies. In their paper in IBIS, Roeland Bom (NIOZ Royal Netherlands Institute for Sea Research) and colleagues from the Global Flyway Network argue that the taymyrensis subspecies should be further divided between those that winter in the Middle East (and presumably East and South Africa) and those that winter in West Africa. Their recommendation is based on studies of migratory behaviour, breeding area, morphology (measurements) and population genetic differentation in mitochondrial DNA. Their main focus has been a study of spatial and temporal differences in migration routes.

How many subspecies?

The most famous subspecies of Bar-tailed Godwit is baueri; these amazing birds fly directly from Alaska to New Zealand at the end of the breeding season and return via the Yellow Sea in spring. Theirs is the most impressive migration route of any wader, including non-stop flights of well over a week. These are the largest Bar-tailed Godwits.

Bar-tailed Godwits wintering in Australia are mainly birds of the menzbieri race. These breed from central Siberia across to north-east Russia and migrate north and south via the Asian coast, especially the Yellow Sea. Bar-tailed Godwits breeding further south in eastern Russia are usually described as anadyrensis, these birds are smaller than baueri but bigger than menzbieri.

Bar-tailed Godwits breeding in western Siberia have all been known as taymyrensis, named after the Taymyr** Peninsula, while those breeding in Scandinavia, Finland and the far west of Russia are identified as lapponica. Nominate Limosa lapponica lapponica birds winter in northwest Europe and most taymyrensis use the East Atlantic Flyway to winter at least as far south as Guinea-Bissau in West Africa. Taymyrensis is the smallest race. Lapponica and taymyrensis were recognised as separate subspecies two decades ago and this differentiation has been useful in discussions about Bar-tailed Godwit conservation. In the UK we see both lapponica and taymyrensis, as discussed in Bar-tailed Godwits: migration and survival.

Until now, birds migrating south from western Siberia, as far as East and South Africa, have been considered to be taymyrensis and it is these birds that have been reclassified – with the new name of yamalensis, after the region of Russia in which the subspecies breeds.

** In the IBIS paper, the authors use Taimyr (rather than Taymyr) as this is preferred by Russian co-authors. I am maintaining consistency within WaderTales.

What’s different?

The studies summarised in the IBIS paper by Bom et al are designed to test whether Pavel Tomkovich, who has studied Russia’s artic-breeding waders for several decades, was right when he suggested that taymyrensis should be split into two subspecies. (See Population structure and migratory links of Bar-tailed Godwits: Current knowledge and unsolved issues in Achievements in Studies on Waders of Northern Eurasia). Based on a small number of movements of ringed birds between areas where there are few birdwatchers, Pavel suggested that L. l. taymyrensis likely comprises two distinct (flyway) populations, one wintering in the Middle East, West Asia and East Africa and breeding on the northern West-Siberian Plain, and the other wintering in West Africa and breeding on and around the Taymyr Peninsula.

Using satellite-tracking, Roeland Bom and colleagues have now described the migration routes, breeding destinations, and annual-cycle timing of Bar-tailed Godwits using wintering areas in the Middle East (Oman) and West Africa (Mauritania and Guinea-Bissau). To understand further the extent to which the two groups are different and whether or not there is any mixing of the two populations, they also examined differences in mitochondrial DNA.

Tracking through space and time

Members of the research team attached solar-powered tags to eastern taymyrensis Bar-tailed Godwits in Oman and western birds in Mauritania, Guinea-Bissau and the Wadden Sea. 52 birds caught between 2015 and 2018 provided usable tracks; 11 birds (12 tracks) linked the western wintering/migration route to the area in and around the Taymyr peninsula, while 9 birds (19 tracks) linked the Middle East to the northern part of the West Siberian plain. To limit potential negative effects of carrying extra weight, only the largest birds were tagged, which means that all of the tags were deployed on females.

Bar-tailed Godwits from wintering areas in the Middle East staged for several days in the areas around the Caspian and Aral Seas, during both northward and southward migration. Both of these areas were already thought to be important for passage waders but analysis of tracking data emphasise the critical role they play in the annual cycle of Bar-tailed Godwits. Some birds also spent short periods of time at sites in the United Arabian Emirates, Iran and India. After leaving their breeding sites, some birds moved up to 1,000 km north, to feed in high-Arctic coastal Siberia, before embarking on southward migration.

Bar-tailed Godwits tracked from wintering sites in West Africa staged in the Wadden Sea, during both northward and southward migration, with other staging points in Spain, Portugal and France, during northward migration. From the Wadden Sea, most West Africa birds flew directly to the northern West-Siberian Plain (near or in the breeding area of the Middle East birds) before heading for the Taymyr Peninsula. All birds routinely moved north before leaving for the Wadden Sea, with many using the same fuelling areas as Middle East birds.

Although birds from West Africa and the Middle East used the same feeding areas in spring and post-breeding, the phenology of the two populations were different. The northward migration, the arrival in the staging and breeding sites and the southward migration of Middle East birds were earlier than for the West Africa birds. Some birds of the two groups could be found in the same areas in Siberia in the pre- and post-breeding staging, emphasising the fact that timing differences can be just as important as spatial separation when it comes to the evolution of subspecies. This ‘spatial overlap but with timing differences’ story is similar to that seen in limosa and islandica Black-tailed Godwits in The UK and The Netherlands, as discussed in Godwits in, godwits out: springtime on the Washes.

Body size and shape

The measurements of taymyrensis and yamalensis overlap but there is a tendency for Middle East birds to have smaller bills and longer wings than West African birds. The authors suggest that, given that the two subspecies have similar diets during the non-breeding season, the different bill structure may be related to feeding requirements during the breeding season, with taymyrensis breeding in open tundra and yamalensis breeding in forest tundra and bogs within the boreal zone. It is interesting to note that the morphological differences between the taymyrensis and yamalensis subspecies are greater than those between taymyrensis and lapponica.

Genetics

West African and Middle East birds could not be separated using genetic tools available to the researchers. The authors point out that there is very little genetic variation between the three more western subspecies (lapponica, yamalensis and taymyrensis), indicating that either these three populations may have diverged recently (i.e. well after the last Ice Age) or that there is still some (small) geneflow between subspecies. The authors dicusss this in depth, in the context of all known populations of Bar-tailed Godwits and of waders in general.

A new subspecies

Roeland Bom and colleagues conclude that the old taymyrensis taxon consists of two distinct populations with mostly non-overlapping flyways, which warrant treatment as separate taxonomic units. They argue that ‘separation in space and time’ can define separate subspecies and that these differences will become apparent before morphological (size and shape) differences develop and long before it will be possible to spot genetic differences.

Breeding locations of tagged Bar-tailed Godwits

Yamalensis Bar-tailed Godwits breed on the northern West-Siberian Plain including the Yamal Peninsula. Birds of the subspecies follow the Central Asian Flyway, with main stopover sites in the Caspian Sea and the Aral Sea. Satellite tracking has connected Oman with other wintering areas in the Middle East, Iran, Pakistan and West India. It is likely that some birds continue further south on the West Asian – East African Flyway, with two, earlier ring recoveries showing that the subspecies can winter as far south as South Africa.

Conservation considerations

One of the key benefits of defining yamalensis as a new subspecies is that priority setting (and hence funding) for conservation action is often defined at the subspecies level. As we learnt from Ashwin Wisvanathan and Les Underhill at the International Wader Study Group conference in 2021, the numbers of waders spending the non-breeding season at the southern extremes of the Central Asian Flyway (in India) and of the West Asian – East African Flyway (in South Africa) have declined alarmingly over the last few years.

Roeland Bom and colleagues quote population estimates for taymyrensis and yamalensis Bar-tailed Godwits of 600,000 and 100,000-150,000 respectively but these are old numbers and West African winter populations are known to be declining. It is important to establish new population estimates and trends for the two populations and better to understand the migration of yamalensis. Potentially, more satellite tracking might help to suggest where to look for wintering yamalensis along the vast coastline from the southern tip of Africa to the southern tip of India, and to identify further spring and autumn stop-over sites.

Paper

The paper in IBIS concludes with a detailed description of the subspecies Limosa lapponica yamalensis, together with measurements that separate it from other races. You can read more in:

Central-West Siberian-breeding Bar-tailed Godwits (Limosa lapponica) segregate in two morphologically distinct flyway populations. Roeland A. Bom, Jesse R. Conklin, Yvonne I. Verkuil, José A. Alves, Jimmy de Fouw, Anne Dekinga, Chris J. Hassell, Raymond H.G. Klaassen, Andy Y. Kwarteng, Eldar Rakhimberdiev, Afonso Rocha, Job ten Horn, T. Lee Tibbitts, Pavel S. Tomkovich, Reginald Victor & Theunis Piersma. IBIS.

Other WaderTales blogs about waders on the West Asian – East African flyway

Well-travelled Ring Plovers makes a link between North Africa and the furthest northeast corner of Russia, extending the reach of the West Asian – East African Flyway further east than sometimes shown on maps.

In search of Steppe Whimbrel describes the migration of Numenius phaeopus alboaxilliaris, a Whimbrel subspecies that has already been declared extinct once. The subspecies migrates between the steppes of Kazakhstan and Russia and coastal East Africa.

Following Sociable Lapwings describes the migration of this threatened species along both the West Asian – East African Flyway and the Central Asian Flyway, with some interesting thoughts about a migratory divide in a wader which exhibits relatively low philopatry.


WaderTales blogs are written by Graham Appleton (@GrahamFAppleton) to celebrate waders and wader research. Many of the articles are based on published papers, with the aim of making shorebird science available to a broader audience.

The flock now departing

“The flock now departing from the tideline is bound for Beauvais. Curlew can change here for destinations in Germany and Russia”. It’s fascinating to wonder what might be happening when a flock of waders takes to the air, gains height and sets off in a particular migratory direction. With more individuals wearing tracking devices, it was only a matter of time until someone would have data that provides clues as to the association of individuals within flocks – as we see in a 2021 paper in Bird Study by Frédéric Jiguet and colleagues: Joint flight bouts but short-term association in migrating Eurasian Curlews.

Setting off on migration

When we get on a plane to a particular destination, everyone else who is on the same journey has chosen to travel at the same time and we all know where we are going. Each of us has checked that we have what we need for the journey and has a plan of what to do when we land – whether that involves a short shuttle to home or a lay-over before catching another flight.

For waders, planning must be more random? It’s presumably safer and more efficient to be part of a flock but how do you know which flock to join, who organises the schedule and is information shared? We can get some clues from observations of departing migratory flocks. In estuaries, there is often the chatter (which is hard to interpret but tells us that something is about to happen), then the first birds take to the air and start to gain height. A few birds may peel off and return to the tide-line while other birds take off and catch up with the departing flock. As the birds gain height, the direction of travel becomes clearer and more birds may decide to return to the mudflats. There is now a migratory flock of birds that are committed to flying in a particular direction. We have no idea how that direction was chosen, of course, but there is a plausible explanation as to how the flock might have formed.

This is not the last decision that members in a flock might need to make. Tired birds may need to drop out of the flock, to take a break. Perhaps some birds might realise that the direction of travel does not work for them and the flock might break up?

It can be just as chaotic when a flock reaches a destination. Watching Black-tailed Godwits arriving in South Iceland in April is fascinating; a tired flock might come in off the sea, land and start drinking, before either resting or feeding, but this is not always the case. On a clear day with fair winds, the flock may split up, with some birds keen to keep flying and others happy to stop. This reinforces the impression that a flock only maintains its integrity as long as being in a group meets the needs of the individuals it contains.

Tracking Eurasian Curlew

Understanding migration is an important element of Curlew conservation studies in France.

In their Bird Study paper, Frédéric Jiguet and colleagues describe four cases of joint migration by tagged Eurasian Curlews. Their observations were a biproduct of research aimed at a better understanding of the origins and migration patterns of Curlew that spend the winter in France. The species has been a popular target for French hunters, many of whom are keen to resume shooting, as you can read in the WaderTales blog Black-tailed Godwit and Curlew in France. It is estimated that more than 7000 Curlew were shot in France annually prior to 2008, when the first moratorium was put in place.

There is an urgent need to understand links between wintering sites and breeding sites, especially in areas where the species is in rapid decline. How important is France to the Curlew that breed in countries such as Poland and Germany? The current ban on shooting is not perfect (see paper in Forensic Science International: Animals and Environments) but it is better than nothing, given rapid declines in Curlew numbers across Europe.

In winter and spring 2020, the research team deployed 61 GPS tags on Curlews in France and Germany, hoping to learn more about breeding ecology and migratory connectivity. In a separate study, in Poland, four captive-bred juvenile curlews were tagged and released in July 2020. Between them, these tagged birds led to four cases of joint migration bouts. One case concerned two adults leaving their wintering ground for the pre-breeding migration. Two other cases were birds leaving their breeding grounds at the start of migration. The last one was of two juveniles initiating their first flights to the non-breeding grounds.

Spring migration

About 27,500 Curlew spend the winter in France (see French report produced jointly by government and shooting groups), representing about 5% of the European population. Tracking has shown that these birds breed in Belgium, Germany, Sweden, Finland, Austria and Russia (see article published by Bird Guides) but there are reports of ringed birds from many other countries, including the threatened populations in Poland and the UK.

Thousands of Curlew spend most of the year in coastal France – representing 5% of the European population.

Return migration to breeding areas takes place in early April. Frédéric Jiguet reports ‘groups of curlews rising high in the sky at sunset’ from the Moëze-Oléron and Baie de l’Aiguillon Nature Reserves in southwest France.

Back-mounted GPS tag

On 17 April 2020, two individuals wearing tracking devices left their French wintering site at sunset, between 22:37 and 22:40. They became closely associated just ten minutes prior to the start of migration, having typically stood 100 metres apart during the previous hour. They flew together for seven hours before making a stop-over north of Paris, between Creil and Beauvais, in the Thérain Valley.

  • 200185 was on its way again two hours later, flew for six hours, stopped again in the Netherlands and arrived in Norderney, an island of the Wadden Sea in northern Germany, at 18:39 on 19 April.
  • 200187 had a much longer layover in the Thérain Valley, making another evening departure at 20:05 on 18 April. It continued migrating, in stages, for more than a month, crossing the Ural Mountains and reaching the Yamalia municipality, in Asian Russia.

Two birds that had been on the same flight from southwest France ended up in very different locations and at very different times. The German Curlew reached its summer destination five weeks before the Russian bird arrived on territory, the latter having secured places on several different ‘international flights’ as it made its way east and north (see figure below).

Post-breeding migration

After breeding, adult Curlew head towards wintering sites, perhaps stopping to moult en route. Some birds do not travel far; for instance, there are colour-marked birds that winter on the Wash (eastern England) and fly just a few kilometres inland to breed. The Bird Study paper includes reports of two occasions when tagged birds have been spotted migrating together from German and French study areas. Southerly migration of all four birds commenced during the evening of 17 June 2020.

French birds: Two individuals departed simultaneously from Deux-Sèvres (central France) between 19:16 and 19:17 for a non-stop southward flight and arrived together at Ria de Treto estuary, in northern Spain on 18 June at 05:49. The two birds departed separately from this stopover site the same day (18 June).

  • 200201 departed at 18:18, for a non-stop flight to Kenitra (Morocco) where it stopped briefly, before moving a short distance north to Merja Zerga.
  • 200204 departed at 19:46 and flew to the Atlantic coast of Spain, stopping for 2.5 hours on Isla Cristina and then flying to its final destination at Ilha de Tavira, in southern Portugal.

After separation, the two birds travelled at different times but followed quite similar routes and even flew at similar altitudes.

German birds: On 17 June 2020, two individuals departed simultaneously from Dollar Bay, in the Wadden Sea National Park. 201075 began migration between 18:58 and 19:03. After five kilometres, if flew over 201072 at an altitude of about 190m. The latter bird took off and joined 201075. They then flew together for five hours, landing in the Rhine-Meuse-Delta (Netherlands).

201075 departed from the Rhine-Meuse-Delta on 20 June and, after one more stop-over, reached its final destination on the Brittany coast on the evening of the next day.

201072 was also bound for Brittany. It departed on 23 June and flew non-stop for six and a quarter hours.

Migration of juveniles

It will be hard to satellite-tag enough wild juvenile waders to pick up instances of marked individuals migrating in the same flocks. However, head-starting may give some clues as to what might happen when naïve flocks of juvenile waders start their migratory journeys, months after the parents have left them. The full story is told in the paper but a quick summary tells us that two Polish head-started Curlews were released on 1 July, departed together on 5 August and landed in the Baie de l’Aiguillon (France) on 8 August. In between times, they came close to landing in The Netherlands, flew along the English coast from Dover to Poole, flew a long way south and west around the Bay of Biscay and then northeast to the coast of France. They both spent the winter in the Baie de l’Aiguillon but not together.

Although it will be difficult to compare the migratory behaviour of wild-caught and head-started wader chicks using satellite tags, just because of probabilities and costs, researchers are building up datasets using smaller geolocators and GPS tags. Here’s hoping that we will soon know more.

Paper

The nutrient-rich mud of Ile Madame

This paper provides observations of just four instances of joint migration but each story is fascinating. They give us insights as to what might be possible as devices get smaller and when land-based tracking stations collect signals from passing birds. For the moment we can use our imagination to interpret the chattering of pre-migratory flocks of waders, the appearance of a small flock of waders at an inland spot in spring and the noisy arrival of a lone Curlew on an estuary in June.

The paper contains a lot more detail about the methods used to collect and interpret data and a discussion that sets Curlew migration within a much broader conceptual context. Here’s a link:

Joint flight bouts but short-term association in migrating Eurasian curlews.

Frédéric Jiguet, Pierrick Bocher, Helmut Kruckenberg, Steffen Kämpfer, Etienne Debenest, Romain Lorrillière, Pierre Rousseau, Maciej Szajdaand & Heinz Düttmann. Bird Study. DOI/10.1080/00063657.2021.1962805

Wintering Curlew from as far away as Russia and Sweden can be found roosting in these French saltmarshes

WaderTales blogs are written by Graham Appleton (@GrahamFAppleton) to celebrate waders and wader research. Many of the articles are based on published papers, with the aim of making shorebird science available to a broader audience.

On the beach: breeding shorebirds and visiting tourists

Rising sea levels, stormier weather, coastal development and more people are putting increasing pressures upon shorebirds that nest on beaches. A paper about the breeding waders of Norfolk & Suffolk (UK) coasts illustrates the importance of understanding human behaviour when trying to maintain (or create) space for breeding plovers. This paper will be of particular interest to conservationists trying to support breeding populations of species such as Ringed Plover, Kentish Plover, Piping Plover and Snowy Plover.

The problems of disturbance

Around the globe, nesting plovers are being threatened by human disturbance. Local initiatives to reduce these impacts include the installation of electric fences, recruiting volunteer wardens and changing local bylaws. You can learn more about which interventions work from Conservation Evidencethere’s more about this at the end of this blog.

Panning out, to look at the bigger picture, is it possible to determine where conflicts between breeding birds and tourists are likely to occur, so that one can try to resolve the problems before they start? Can this information help to inform planning decisions?

Oystercatcher with chick

Ringed Plovers and Oystercatchers

The coastline of Norfolk and Suffolk should be an ever-changing environment, dominated by sand and shingle beaches. Coastal defences artificially maintain the barrier between sea and land but sea-level rise is predicted to over-top and destroy sea walls during storm events. How will East Anglia’s beach-nesting waders cope with squeezed beaches at the same time as East Anglia is seeing increasing tourist numbers? In a 2020 paper in Global Ecology & Conservation, Jamie Tratalos and colleagues from the University of East Anglia investigated the distributions of nesting Ringed Plovers and Oystercatchers around the beaches of Norfolk and Suffolk, relating settlement patterns to visitor numbers.

Snettisham beach – sunny weather is not great for the local breeding Ringed Plovers

Ringed Plovers and Oystercatchers breed on sand and shingle beaches that are also attractive to people. They are prone to disturbance, especially by dogs that are allowed to run free, as discussed by Gómez-Serrano (2021). Nests can be trampled, incubation can be interrupted and chicks can be killed. Liley and Sutherland (2006) showed that, over a 9 km stretch of Norfolk coastline, Ringed Plovers bred less successfully when exposed to disturbance by beach visitors, and population declines in this species have been linked to human disturbance (Birds in England by Brown & Grice, 2005). Human recreation has also been shown to be associated with reduced breeding success in Eurasian and other oystercatcher species (Tjørve & Tjørve, 2010), and Ens and Underhill (2014) suggest that increased human use of the coastal zone, combined with increased risk of nest flooding and loss of wetlands, may threaten the conservation of oystercatchers around the world.

Ringed Plovers can have several breeding attempts in the course of a summer

UK breeding populations of Ringed Plover have declined in recent decades, from a conservative estimate of approximately 8400 pairs in 1984 to 4070 in 2007 (Conway et al., 2019) and the species is now red-listed (see the WaderTales blog: Nine red-listed UK waders). Oystercatchers have undergone considerable Europe-wide decline in recent decades and the species has been classified as ‘Near Threatened’ globally (IUCN, 2020).

Counting birds and people

In 2003, when the study at the heart of the Tratalos paper was carried out, East Anglia’s beaches between the Wash and the River Stour held about 3% of the UK’s breeding Ringed Plovers, as well as relatively small numbers of breeding Oystercatchers. As part of a bigger climate change research programme, Tratalos et al were keen to understand what drove the distribution of Ringed Plovers and Oystercatchers, in order to be able to include conservation actions in plans to manage the changing coastline of Norfolk and Suffolk, especially associated the abandonment of outer sea defences. Their research was written up in a 2020 paper in Global Ecology & Conservation.

In the study, the authors examined a 212 km stretch of coastline, mapping all breeding pairs of Ringed Plover and Oystercatcher, as well as the environmental characteristics of beaches. Data on the location of bird territories, and the habitats in which they were found, were collected by Dave Showler in the period between early April and mid-June in 2003. Details of survey methods can be found in the paper.

Map data from Bird Atlas 2007-11 (BTO, BirdWatch Ireland and SOC)

Visitor numbers to different beaches were assessed by filming from a light aircraft, flying at an altitude of 150 metres. 38,634 human visitors were mapped from three flights during sunny weekends in April, June and August, when the tide was at approximately mid phase. There were pronounced peaks in visitor numbers along the coastline, with 19 of the 1003 beach sections experiencing over 10 times the average number of visitors and 231 sections hosting none.

The key findings from surveys and analyses were:

  • Of just over one thousand 200m sections of beach surveyed, 183 beach sections contained Ringed Plover territories (266 breeding pairs) and 117 contained Oystercatcher territories (223 pairs).
  • There were more occupied territories in less-visited areas, for both species. See table relating the visitor index to occupation of sectors. An index of 0.13 means that visitor numbers were 13% of the mean across all sectors.
  • No Oystercatchers were found breeding in sectors where the visitor index was higher than 2.8. No Ringed Plovers were found in sectors where the index was above 5.5.
  • Ringed Plovers territories were more common in sections that had dunes at the back of the beach and where the beaches were broader at low tide.
  • Oystercatchers appeared to need space above the high-water mark, as well as a broad intertidal area.

The associations between territories and habitat enabled the team to predict the number of pairs of waders that might have been present in areas which were highly impacted by visitors. If visitor numbers were reduced to zero across the whole study area, breeding potential could be hugely increased.

Feeding on the mud at low tide – Ringed Plovers and Oystercatchers need a broad intertidal area
  • The authors predict that there would have been an additional 90 beach sections where Ringed Plovers could potentially establish territories, suggesting that tourism and the local use of beaches has already removed 33% of Ringed Plover breeding habitat.
  • There were 96 sections where breeding Oystercatchers might have been expected to be found, so they have already lost 45% of potential habitat.

Practical considerations

These results suggest that human activity on beaches influence the location of breeding territories of Ringed Plovers and Oystercatchers, with both species using territories where the number of human visitors was relatively low, when considered both at the scale of the whole Norfolk and Suffolk coast, and locally within areas of this coastline.

In the absence of people, there appear to be clear features of the areas that determine if sectors are used by both wader species for breeding. This makes it possible to predict places where increased access could cause problems so that, ideally, tourism might be encouraged in areas that are less likely to be used by breeding waders. Unfortunately, the beaches that are great for red-listed Ringed Plover – with a back-drop of sand dunes, a sandy beach to walk along at high tide and a gently-shelving intertidal area – are also ones that attract people. This makes it harder to create discrete ‘people zones’ and ‘wader zones’ than might otherwise be the case.

Access points create issues for breeding wader but the effects of most visitors are localised. Unless birdwatching or exercising a dog, the typical tourist will not stray more than 300 metres from a carpark, as indicated in the graphic above. In less-disturbed beach sections, where Oystercatchers set up territories, chicks can hide in upper-beach vegetation until parents indicate that it is safe to come out to be fed.

When Emma Coombes (Global Environmental Change, 2009) asked visitors to Norfolk’s beaches what they were looking for, there was a remarkable consistency in the responses from dog-walkers, sun-bathers and birdwatchers. They all wanted to be on remote flat, sandy beaches, with sand dunes. They would appreciate a car park and toilets too. As soon as such facilities are provided, of course, visitor numbers increase, the remoteness is lost and so are breeding waders, unless fences and wardens are introduced.

Winterton-on-Sea beach is promoted as a tourist location with access to a national nature reserve

Planners have few tools available to them, when it comes to protecting stretches of coastline. All that they can control is development (e.g. new roads, housing and tourist accommodation) and facilities such as car parks and toilets. This paper clearly shows the need to understand the local features that are needed by nesting waders and the importance of documenting current distributions, so that local and national planning authorities have the information they need when planning for the future.

Paper in Global Ecology & Conservation

Vulnerable Ringed Plover chick

Regional models of the influence of human disturbance and habitat quality on the distribution of breeding territories of common ringed plover Charadrius hiaticula and Eurasian oystercatcher Haematopus ostralegus. Jamie A. Tratalos, Andy P. Jones, David A. Showler, Jennifer A. Gill, Ian J. Bateman, Robert Sugden, Andrew R. Watkinson & William J. Sutherland.

Conservation evidence

Before trying a new conservation technique on a local patch, it is worth checking out what has been tried elsewhere. A quick visit to the Conservation Evidence website and a search on ‘beaches’ and ‘bird conservation’ produced a list of 26 actions that have been written up in papers or grey literature. Although many of these interventions are more appropriate to tern conservation than shorebird conservation, a few seem to be particularly relevant to people who are considering how to help breeding Charadrius plovers. Five potential actions are assessed as ‘likely to be beneficial’

  • Use signs and access restrictions to reduce disturbance at nest sites
  • Physically protect nests with individual exclosures/barriers or provide shelters for chicks of waders
  • Protect bird nests using electric fencing
  • Physically protect nests from predators using non-electric fencing
  • Physically protect nests with individual exclosures/barriers or provide shelters for chicks of ground nesting seabirds
Dogs cannot read ‘no entry’ signs, designed to save space for breeding waders and terns

The Conservation Evidence website aims to make scientific research available to conservation practitioners. Anyone considering any of the interventions listed above can see a quick synopsis of what worked (and what didn’t work) in which circumstances. Anyone who has discovered another successful management technique is urged to write up their study – so that it can be added to the database.


WaderTales blogs are written by Graham Appleton (@GrahamFAppleton) to celebrate waders and wader research. Many of the articles are based on published papers, with the aim of making shorebird science available to a broader audience.

Remote monitoring of wader habitats

In a 2021 paper in Basic and Applied Ecology, Triin Kaasiku and colleagues use radar remote sensing to assess the continued suitability of large areas of Estonia for breeding waders. Although the focus of the paper is upon the performance of an agri-environment scheme, the results are of broader relevance, as conservation biologists seek to monitor vegetation growth and the encroachment of shrubs and trees, especially associated with a warming climate, afforestation and farmland abandonment.

Ground-nesting waders

Many species of waders breed in open habitats. Dense vegetation can remove feeding and nesting areas, and shrub and tree encroachment provides shelter for predators. Previous WaderTales blogs have discussed the perceived and actual predator effects of patches of woodland on breeding Lapwing populations and the difficulty of removing trees and predators that have been planted in peatland (Trees, predators & breeding waders). At a larger scale, vegetation growth, trees and shrubs are threatening breeding areas in arctic and subarctic regions, as discussed by Tómas Gunnarsson in Losing space for breeding waders. Locally, changes to land management, such as reduced sheep grazing and the abandonment of grouse moors in the British uplands, may have unintended consequences for breeding waders, including Curlew.

In Estonia’s coastal grasslands, horses are part of the grazing regime

Maintaining Estonia’s coastal grassland

Estonia’s farmed, coastal grasslands are semi-natural habitats that are vital for a range of species, particularly breeding waders. They also act as soft barriers that dissipate wave energy along coastlines and capture carbon. Across the European Union, the importance of these areas has been recognised, resulting in agri-environment schemes (AES) that preserve farm incomes while encouraging nature-friendly management methods that are associated with lower yields. 

Redshank nest in a clump of grass

Getting the balance right is tricky – for example, too much grazing can reduce heterogeneity and impacts nesting waders (see Big Foot and the Redshank nest), but too little grazing can mean that coastal marshes become rank, as tall grasses replace low-growing plant communities and scrub and trees start to encroach. In a previous paper – Managing coastal grasslands for an endangered wader species can give positive results only when expanding the area of open landscape – Triin Kaasiku, Riinu Rannap & Tanel Kaart emphasised the importance of open, wet grasslands for breeding Dunlin.

The main objective of coastal grassland management in Estonia, in addition to supporting local farming communities, has been to preserve the breeding habitat of several threatened wader species, particularly Dunlin, Ruff, Black-tailed Godwit and Redshank. Nine of the eleven species that breed in these areas have declined moderately or strongly since 1980. Population changes are believed to have been largely driven by habitat loss and increased predation. These problems are recognised around the whole Baltic coast, which is also experiencing more frequent summer storms and flooding. See Dunlin: tales from the Baltic.

Remote sensing

Cute Curlew chick

The Kaasiku team used freely-available European Space Agency Sentinel-1 Synthetic Aperture Radar (SAR) images to assess habitat suitability of coastal grasslands, as breeding sites for waders across Estonia. Although this technique has been used before on cut grasslands, it had not been tested on less uniform grazed areas. The aims of the project were to:

  • Determine the ability of SAR satellite images to classify suitable and unsuitable areas for breeding waders.
  • Compare the habitat suitability for breeding waders under different management schemes, across over 200 km2 of Baltic coastal grassland habitat.
  • Propose methods to improve coastal grassland management.

One of the key parts of the project was to be able to ground truth the SAR data, by visiting sites being managed in different ways (or not at all) in order to collect direct measurements and make observations relating to the suitability of habitat for breeding waders. Details of the methods used are given in the paper.

Research findings

Using SAR radar images, it was possible to correctly identify 88% of suitable habitat and 74% of unsuitable habitat, using a classification boundary that was created using a test set of the data.

Grazing maintains open habitats (left and centre) and restricts reed growth (right)

With renewed confidence, the research team was able to assess whether management prescriptions are actually delivering the desired habitats – for waders and for biodiversity. Their results are encouraging – these agri-environment schemes are broadly working:

  • 43% of Estonia’s coastal grasslands are in a favourable state, in terms of vegetation height.
  • The proportion of suitable habitat is higher (60%) in areas where basic-level conservation subsidies are paid.
  • The highest proportion of suitable habitat (76%) is achieved in areas receiving a top-up conservation subsidy.
  • Long-term grazing ensures better habitat quality: in areas where a basic subsidy has been paid for at least the last five years, 65% of the habitat was classified as suitable.
  • The conclusions were tested by the breeding waders themselves – 98% of the 268 nests located by the team were found in areas classified as suitable.

Although the use of radar remote sensing provides a way of assessing the general suitability of grassland for nesting wader community, it does not provide information on the availability of species-specific microhabitats, finer-scale vegetation characteristics, or soil moisture, all of which are important to breeding waders.

Effectiveness of subsidies

Baltic Dunlin (schinzii) need help

In Estonia, the area of coastal grassland under conservation management has increased from 8,000 to 11,500 ha since the implementation of the AES scheme in 2001. Payments account for 40% of farmers’ incomes. While the payments ensure 60% of the habitat is managed suitably, the remaining 40% does not create habitat for breeding wadersthe main target of the AES for coastal grasslands. The authors suggest that two decades of minimal oversight of the outputs of the scheme has led to sub-optimal delivery. This is unsurprising, given the cost of field visits to farms. Perhaps satellite imagery will help in the future, especially if used five years after the onset of conservation management, when the cumulative effects can be detected.  These assessments will also enable top-up payments to be focused upon areas where 75% of the shoreline is free of high vegetation.

The authors note that the current subsidies are not associated with prescribed stocking rates and suggest that this issue needs to be addressed. Using the right levels of grazing at the right time of year in the right places can make a big difference to breeding waders, as mentioned earlier and discussed in Dunlin – tales from the Baltic and the UK-focused blogs Big Foot and the Redshank nest and Redshank – the ‘warden of the marsh. T

Nest-trampling rates seem low in Estonian coastal grazing habitat, suggesting that there may be scope to increase grazing pressure, especially if cattle are only released onto coastal grasslands in late May.

A broader context

Ruff nest in Estonia

While fencing out predators might be a good way to support breeding waders nesting at high densities in nature reserves or on intensively managed sites, landscape-level support is going to be needed if we are to conserve thinly distributed European species such as Curlew, Dunlin and Golden Plover. Across Europe, prescriptions are in place to adjust grazing levels so as to suit breeding waders but it is difficult to assess the effectiveness of these measures or even to judge whether management is taking place at the right scale and intensity. Triin Kaasiku and collegaues have demonstrated that remote sensing can be a cost-effective way to monitor the delivery of conservation measures indirectly, although they point out that this does not replace detailed monitoring of bird communities. This work has broader applicability, as we seek to measure global-scale habitat changes that will affect increasingly beleaguered wader populations, nesting in some of the most remote parts of the globe.

The paper at the heart of this blog is:

Radar remote sensing as a novel tool to assess the performance of an agri-environment scheme in coastal grasslands. Triin Kaasiku, Jaan Praks, Kaidi Jakobson & Riinu Rannap. Basic and Applied Ecology. July 2021.


WaderTales blogs are written by Graham Appleton (@GrahamFAppleton) to celebrate waders and wader research. Many of the articles are based on published papers, with the aim of making shorebird science available to a broader audience. A catalogue of blogs is available HERE.

 

Flying high with Great Snipe

As tagging devices get smaller and more sophisticated, they are revealing even more wonders of shorebird migration. We already know that Great Snipe are amazing – flying up to 7,000 km non-stop – but a 2021 paper by Åke Lindström and colleagues describes a striking daily cycle of altitude change during their long migratory journeys.

In their new paper in Current Biology, Åke Lindström and colleagues have used activity and air pressure data from multisensor dataloggers to show that Great Snipes repeatedly changed altitudes around dawn and dusk, between average cruising heights about 2,000 m (above sea level) at night and around 4,000 m during daytime. Most birds regularly flew at 6,000 m and one bird reached 8,700 m, an altitude that is just 150m short of clearing the top of Mount Everest! The same daily cycle was apparent everywhere – independently of climate zone, habitat and the height of the land being overflown. Wherever they are, as morning breaks migrating Great Snipes gain altitude – but why?  

Great Snipe

Great Snipe spend the winter in Africa, between 20 degrees south and 15 degrees north of the equator, heading north in spring to breeding areas in Scandinavia and northern Europe, as far east as 95°E (which is the same longitude as Myanmar).

The first paper to reveal the remarkable migration of Swedish Great Snipe appeared in Biology Letters in 2011, when Raymond Klaassen, Åke Lindström and colleagues revealed the tracks from three birds carrying geolocators. During these journeys, these individuals made long and fast autumn movements, covering between 4300 and 6800 km in two to four days and overflying suitable stopover sites that were used in spring. Ground speeds of 15 to 27 m/s are equivalent to between 54 and 97 km/h (33 to 60 miles per hour).

In 2016, using 19 tracks from four years of captures and recaptures, the same team published a paper in the Journal of Avian Biology. About half of the birds flew directly from the breeding grounds to sub-Saharan Africa, the others making a few shorter flights down through Scandinavia before embarking on a long trans-Sahara flight. Birds took advantage of wet seasonal conditions in this Sahel region for three weeks before moving south to the Congo Basin. Spring migration consists of a rapid movement across the Sahara, only a little shorter than the very long non-stop autumn fights, followed by slower movements through Eastern Europe. Birds arrived back on breeding areas in mid-May.

Great Snipes spend about eight months of the year in sub-Saharan Africa. In a 2017 paper in Wader Study, Edouard Debayle, Åke Lindström and colleagues analysed the moult and fattening patterns of over 3,000 hunted birds, to try to learn more about the phenology of migration. They discovered that:

  • Adult males arrived in Africa from mid-August, having started and suspended the moult of the main flight feathers before arrival.
  • Females on average arrived somewhat later and were about one month behind in the progress of flight feather moult.
  • The adults of both sexes resumed primary and secondary moult immediately upon arrival and typically completed it by the end of November, in males, and the end of December in females. Juvenile Great Snipes arrived later than adults and did not moult their flight feathers in the first autumn/winter.
  • Males apparently departed northwards between late March and late April, and the females about two weeks later. There is information about rates of fat deposition in the paper.
Great Snipe watches on as team members set a mist net

Flying high

Processing the catch

Several factors could influence the flight altitude of migratory birds. For example, how high is the land over which birds fly, what are the best temperatures and humidity conditions for efficient flight, at what height can a bird find the most helpful winds, can a bird use navigation landmarks and how can predation best be avoided?

Recent tracking of migratory birds of a range of species has shown that individuals change flight altitude more commonly and dramatically than previously thought but why? In their paper in Current Biology, Åke Lindström and colleagues reveal information from 25 tracked Great Snipe journeys and discuss the reasons that may lie between the patterns that they see.

Methods

Information about behaviour and flights was collected from multisensor dataloggers, consisting of an accelerometer for activity measurements, a barometric pressure sensor with internal temperature sensor, a light-level sensor, a real-time clock, and memory. The dataloggers weighed 1.4–1.7 g (about 1 % of a bird’s total body mass) and were attached to a plastic ring on the bird’s tibia.

In total, 107 dataloggers were put on Great Snipes between 2015 and 2019. Of these, 36 birds were retrapped one, two or three years later (but four birds had lost their loggers). This is an overall recapture rate of 34%, which is similar to the figure for ringed birds. In total, 25 out of the 32 retrieved loggers had functioned for some time, and 16 carried information on flight altitude for at least one of the long flights.

The methods section provides full details of how geolocator data were interpreted and altitudes were calculated and adjusted. Information on air temperatures at different altitudes and the topological features on flight paths provided a background against which to try to understand migration patterns.

How high?

The new data from a small number of multisensor data-loggers greatly enriched the migration story of Swedish Great Snipe, as revealed byÅke Lindström and colleagues. The key results in the Current Biology paper are:

Breeding habitat in Sweden

Flight duration: There were three long flights, two legs on the way south and one on the way north. As noted earlier, northerly migration slowed once birds landed in Europe.

  • On average, male Great Snipe left Sweden on 24th August and flew across Europe and the Sahara for 73.4 hours, before landing in the Sahel.
  • The mean departure date from the Sahel was 24th September. An average of 23.2 hours later a bird would land in the Congo basin.
  • Northerly flights commenced on 18th April, lasting an average of 82.4 hours and concluding in Europe.

Cyclic flight altitudes: There was an overall strong and consistent daily cycle in the altitudes used by the Great Snipes, in all three long flights. After a night at moderate to high altitudes the birds ascended to very high altitudes in early morning, stayed at these levels during the day, and descended again in late afternoon. They then repeated this cycle for one or two more days.

  • The mean individual daytime flight altitude in the first Autumn flight was on average 4,549 m, compared to 2,126 m at night.
  • For the In-Africa autumn flights, comparative figures were 3,874 m in daytime and 1,860 m at night.
  • For the Spring flights, comparative figures were 4,114 m in daytime and 1,612 m at night.

These altitudes were estimated from air pressure readings and may be underestimates. For comparison, the highest point in the Alps is 4807 m.

Peak altitudes: Some Great Snipes occasionally flew extremely high and then always during daytime. Three birds in Autumn and two birds in Spring reached 7,000 m or more. Migratory waders are able to carry out flapping flight at such high altitudes due to several physiological adaptations of the heart, lungs and muscles. The single highest altitude estimate of 8,077 m was reached in Autumn. If air pressure is accounted for this bird may have been flying above 8,000 m for five hours, perhaps reaching an altitude of 8,700 m, and coping with an air temperature of -21.3 °C. Putting this in a local context for readers: Mount Everest is 8848m high and the high points in other regions of the world are: Africa (5895 m Kilimanjaro), South America Aconcagua 6959 m, North America Denali 6190 m and New Zealand Mt Cook 3754 m.

Ambient temperature, wind condition and humidity appeared not to influence the differences in day and night altitudes chosen by Great Snipes.

Discussing the results

Great Snipe with a datalogger

The daily pattern of altitude changes for Great Snipes was very similar between Autumn, In-Africa and Spring flights, suggesting a common cause that is largely independent of climate zone (temperate or tropical), topography and landscape overflown (forest, savanna, farmland, desert or water). Altitude changes have been reported for other waders, such as Black-tailed Godwits, that have been linked to both ambient temperatures and finding more beneficial winds.

The authors discuss the possibility that landmarks are easier to see from a higher altitude when flying in daylight and suggest that predator avoidance may also account for higher day-time elevations. It would be interesting to know if a daily cycle is apparent in long flights over areas largely lacking bird predators, such as vast oceans.

There is no daily cycle in ambient air temperature or wind conditions at high altitudes that could explain the overall regular pattern of flight altitude selection found in Great Snipes but the authors discuss the theory that the warming effect of solar radiation may be countered by flying through colder, higher air. The temperatures at these heights would be too cold at night.

There are still few papers that provide altitude data for long-distance migrating birds but all of them report altitude changes and have revealed some surprisingly high flight altitudes. With more studies we may well find that migration is even more impressive than we already thought!

Paper

The full methods, results and discussion can be read in the paper in Current Biology.

Extreme altitude changes between night and day during marathon flights of Great Snipes Gallinago media: Åke Lindström, Thomas Alerstam, Arne Andersson, Johan Bäckman, Peter Bahlenberg, Roeland Bom, Robert Ekblom, Raymond H. G. Klaassen, Michał Korniluk, Sissel Sjöberg & Julia K. M. Weber.


WaderTales blogs are written by Graham Appleton (@GrahamFAppleton) to celebrate waders and wader research. Many of the articles are based on published papers, with the aim of making shorebird science available to a broader audience.

Dunlin: tales from the Baltic

Veli-Matti Pakanen, Kari Koivula  and colleagues have been studying Finnish Dunlin for nearly twenty years. These are schinzii birds that breed in coastal grazing meadows around the Baltic Sea. Several papers have been published, as you will see below, some of which are based on information collected using geolocators attached to leg-flags. Dunlin numbers in coastal Finland are dropping quickly, so this research is important to the conservation of the species. In a 2020 paper, Veli-Matti and colleagues ask whether intensive geolocator-based studies are having a negative effect on individual birds in this already-declining study population.

Global Dunlin

Up to ten races of Dunlin have been identified, which together encircle the globe. Most Dunlin spend the non-breeding season in the northern hemisphere and all migrate north in spring. The breeding and wintering ranges of the various races are summarised at the end of this blog. Here, the focus is on Baltic schinzii Dunlin, a small part of a race that largely winters in coastal North Africa and breeds in southern Scandinavia, Northern Europe, the British Isles, Iceland and southern Greenland.

According to Wetlands International, there were between 4.3 and 6.8 million Dunlin worldwide in 2015, with about one million breeding birds within Europe (BirdLife International). Whilst it is acknowledged that numbers are declining, the large range of the species means that it is still considered to be of ‘least concern’.

Different populations are faring differently. In his description of the changing Dunlin distribution in European Breeding Bird Atlas 2, John Calladine points out that there have been major losses across Europe, including Britain & Ireland, and that Baltic populations ‘are now considered threatened’. The Baltic schinzii population was most recently estimated as between 500 and 640 pairs – less than a fifth of the estimate in the 1980s. John highlights gaps in knowledge that research by the team in Finland and other groups elsewhere are helping to fill.

Migration and survival of schinzii

Dunlin that visit the UK are mostly of the schinzii and alpina races, as indicated in this map. There has been a noticable drop in the number of schinzii birds stopping off in July.

In pre-geolocator days, Ole Thorup and colleagues analysed recovery information available for Dunlin breeding around the Baltic, using 40 years of ringing data from Finland, Sweden, Denmark and Germany. At that stage there were only six mid-winter recoveries in N and NW Africa. The analysis emphasised the importance of wintering and stop-over sites in the Baltic, the Wadden Sea, SE and S England, the Atlantic seaboard of France, and the Iberian Peninsula. Paper in Ardea (2009).

Pakanen et al investigated changes in the survival rates of schinzii Dunlin nesting in Denmark, based on ringing and recaptures of a marked population. They found that annual apparent survival rates dropped from 0.817 to 0.650 between 1990 and 2006, equivalent to a doubling of the chance of dying in any given year. Paper in Bird Study (2016). The importance of monitoring survival rates is discussed in the WaderTales blog: Measuring shorebird survival.

Nests in these flat coastal marshes along the Baltic coast are increasingly susceptible to summer flooding

The use of geolocators enabled Veli-Matti Pakanen to add more detail to the migration story in the 2018 paper, Migration strategies of the Baltic dunlin: rapid jump migration in the autumn but slower skipping type spring migration. He and his colleagues showed that autumn migration is faster than spring migration, characterised by fewer stationary periods, shorter total stopping time and faster flight. The Wadden Sea was found to be an important autumn staging area for all of the tagged birds. Some birds stopped once more before reaching Mauritania. On spring migration, more sites were visited on the way north. The important conservation message from the paper is that Baltic Dunlin may be especially vulnerable to rapid environmental changes at their staging and wintering areas. (In Travel advice for Sanderling there is a suggestion that annual survival is relatively low for birds that winter in Mauritania).

In a 2015 paper in Ornis Fennica, Pakanen et al reported on the results of a single-year analysis of survival rates, concluding that there were no strong effects of leg-flagged geolocators on return rates or reproduction in schinzii Dunlin. However, they did suggest that “long term evaluations that capture the full suite of environmental conditions and assess impact on brood care are needed”. This is a question which leads us neatly on to the 2020 paper: Survival probability in a small shorebird decreases with the time an individual carries a tracking device.

Long-term tracking of individuals

To understand the range of pressures that migratory birds face, one needs to know where individuals spend their time, as was demonstrated in Spoon-billed Sandpiper: Track & Trace and Teenage Waders. Waders of a range of species have been wearing geolocators for ten or more years now, either the same devices on birds which have evaded capture, or a series of tags, when data have been downloaded and replacement tags fitted. These long-term surveys are developing our understanding of the repeatability of migratory behaviour and how birds deal with variable weather patterns, but is there a long-term cost to the individual birds that are tasked with finding out this important information?

This Dunlin wore a ring-mounted geolocator for one year, to collect movement data

Effects of tracking devices on survival are generally considered to be small. However, most studies to date have focused on birds that were caught in one breeding season and recaptured in the following season, to retrieve the geolocator and download the data. In their 2020 paper, Veli-Matti Pakanen and colleagues were able to investigate the possible accumulation of negative effects when individuals have carried the tracking devices for longer periods. Survival rates for tagged birds were compared with 338 colour-ringed birds that were followed for all or part of the period 2002 to 2018.

In the summers of 2013 & 2014, fifty-three adult schinzii Dunlin were fitted with leg-flag mounted geolocators, with a mass equivalent to 1.5 – 2.0% of body-mass. Of these birds, 17 tags were retrieved after one year and 9 after two years. Other marked birds could not be caught and carried their tags for longer periods (3 or 4 years). The research team found that Dunlin carrying a geolocator had reduced chances of survival. Their models suggest that annual survival of colour-ringed males was 0.813. For a bird that carried a geolocator for a year, annual survival probability declined to 0.748 and to 0.581 for birds that carried the geolocator for at least 2 years. Their data suggest that the reduction in survival rates was greater for females than males, even though females are larger than males.

Summer flooding of coastal breeding area is becoming an increasing problem, and likely to get worse with sea-level rise and more chaotic weather patterns

In a thorough Discussion, the authors consider reasons why tags on small waders may be reducing survival, either through ongoing stresses, impacting on things such as feeding efficiency and the energy needed during migration, or because the extra burden means that tagged birds find it harder to cope with occasional periods of tough environmental conditions. They comment on the condition of the skin under removed geolocators – something that other researchers night want to look out for.

As anyone studying breeding waders will know, nest-trapping to retrieve tags is not easy, especially if adults lose their clutches when incubation has only just started, due to flooding, predation etc. Birds may end up carrying tags for longer than intended. The authors “recommend that the detrimental effects of tagging may be avoided by developing attachment methods that are automatically released after one year, e.g. biodegradable materials”.

Balancing costs and benefits

The results from the Pakanen study of long-term survival suggest that requiring a small wader to carry a geolocator for several years may have an impact on survival. As in all mark-recapture studies, researchers are urged to assess the costs to the individual when seeking to understand what might be affecting the viability of a population.

Four previous WaderTales blogs have discussed tag effects:

Details of the Dunlin tagging effects study:

Survival probability in a small shorebird decreases with the time an individual carries a tracking device.

Veli-Matti Pakanen, Nelli Rönkä, Thomson Robert Leslie, Donald Blomqvist, Kari Koivula. Journal of Avian Biology (2020): https://doi.org/10.1111/jav.02555

Four other papers relating to this Finnish Dunlin study

Grazing by cattle is an important management tool in coastal meadows. Pakanen et al studied the impact of trampling on artificial nests and concluded that even recommended stocking rates were too high for chick numbers that could deliver a sustainable population. Paper in Biodiversity and Conservation (2011). (Redshank on British estuaries are similarly vulnerable to trampling – see Big Foot and the Redshank Nest).  

In a follow-up paper in 2016, Pakanen et al concluded that Dunlin populations could be sustained in grazed coastal meadows as long as there was no active grazing before 19 June. Meadows with grazing cattle attracted breeding birds but there was insufficient breeding success for sustainability. Paper in Ecology & Evolution (2016).

If these schinzii Dunlin chicks return to breed they are likely to try to nest nearby; something that needs to be considered when considering conservation measures

Dunlin are strongly philopatric, with both male and female chicks recruiting to suitable habitat close to natal sites. In a paper in Ibis (2017), Pakanen et al show that natal dispersal of Dunlin is strongly linked to the size of their natal site and how isolated the site is. They suggest that inbreeding may be avoided by creating a network of suitably sized patches (20–100 ha sites), no more than 20 km apart from each other. These may work as stepping stones for recruiting individuals. These results are corroborated by a 2021 microsatellite study in BMC Ecology and Evolution which shows genetic differentiation and isolation by distance within the Baltic Dunlin population.

Up to ten races

In western Europe we see three races of Dunlin – alpina, arctica and schinzii. Wintering birds are almost exclusively of the alpina race, which head north and east to northern Scandinavia, Russia and Siberia in spring. The other northern race is arctica, a very small Dunlin that breeds in low numbers in NE Greenland and possibly Spitzbergen. Schinzii has a very large breeding range, spanning the Baltic, southern Scandinavia, Northern Europe, the British Isles, Iceland and SE Greenland. There is huge variation in the timing of breeding of schinzii, as birds do not return to breeding sites in SE Greenland until the end of May, at the same time as schinzii Dunlin being studied by Veli-Matti Pakanen and colleagues in Finland will have their first young chicks.

Further east, centralis Dunlin replace alpina. Many of these birds use the Central Asian Flyway. Further east still, we find sakhalina that use the East Asian/Australasian Flyway (EAA). Two other subspecies have been identified breeding within the EAA Flyway, the more southerly kistchinski birds and actites, which breeds furthest south, on the Russian island of Sakhalin, on a similar latitude to the UK.

It is generally accepted that there are three Dunlin subspecies in North America. In autumn, arcticola head west from northern Alaska and NW Canada and follow the EAA Flyway, pacifica fly south along the Pacific coast from SW Alaska and hudsonia migrate from central northern Canada using the Atlantic Americas Flyway.


WaderTales blogs are written by Graham Appleton (@GrahamFAppleton) to celebrate waders and wader research. Many of the articles are based on published papers, with the aim of making shorebird science available to a broader audience.