Do population estimates matter?

blog top godwitsThis month (March 2019) saw the publication of Population estimates of wintering waterbirds in Great Britain, which includes all the wader species from Little Stint to Curlew. Given that the Wetland Bird Survey already covers about 2000 wetlands and provides annual monitoring, why do we need to know the total number of birds in Great Britain?

I suggest four reasons:

  • If we count the number of Curlew and we have a figure for the European population then we know that Great Britain is responsible for nearly 20% of Europe’s Curlew each winter, thereby strengthening the case for national conservation action;
  • If we have a national figure, then we know that a flock of 2000 Black-tailed Godwit represents (as it turns out) over 5% of the British total, which is a useful criterion when assessing the conservation importance of individual sites;
  • blog GKPopulation totals help to put annual percentage changes into context;
  • And simply because people ask questions such as “how many Greenshank are there in the country during the winter?”

So, here’s the bottom line. In their 2019 review of waterbird numbers in British Birds, a team from BTO, WWT, JNCC & RSPB reveal that an estimated total of 4.9 million waders spend the winter in Great Britain. That’s about one third of all waders on the East Atlantic Flyway. Impressive!

Please note that Northern Ireland figures are included in an upcoming report for the island of Ireland.

Making the counts

The population estimates owe a lot to those who undertake monthly Wetland Bird Survey (WeBS) counts on estuaries, lakes and waterways, during the winter months, year in and year out. Counts from the period 2012/13 to 2016/17 are used in the population estimates that form the basis for the 2019 review. WeBS data have many other uses, as you can read here: Wetland Bird Survey: working for waders.

blog CUFor species of wader that also make use of the open coast, the Non-estuarine Waterbirds Survey of 2015/16 (or NEWS III) provided additional data, updating the NEWS II figures from 2006/07. The vast majority of our wintering Purple Sandpipers are found on open beaches and rocky shores, as well as large numbers of Turnstone, Ringed Plover and Sanderling, together with significant numbers of Oystercatcher, Curlew and Redshank. There’s more about NEWS in this slightly dated blog: NEWS and Oystercatchers for Christmas.

The last assessment of winter wader populations was made by the Avian Population Estimates Panel and published in British Birds in 2013 as Population estimates of birds in Great Britain and the United Kingdom (APEP3). In here, estimates for waders were largely based on WeBS data for the period 2004-09 and NEWS II. The new assessment is presented as Population estimates of wintering waterbirds in Great Britain and also published in British Birds. It uses WeBS information for the period 2012-17 and NEWS III data. Effectively, there is an 8-year or 9-year difference between the two sets of figures.

The biggest losers

blog graphicGreat Britain is extremely important in the context of the East Atlantic Flyway, as is obvious from the fact that the area holds nearly five million waders. The WeBs counts already monitor the ups and downs on an annual basis but this review provides an opportunity to turn the percentages into actual numbers. It is concerning that, over a period representing less than a decade, the average maximum winter count for six of the species that were surveyed dropped by a total of over 150,000. These big losers were Knot, Oystercatcher, Redshank, Curlew, Grey Plover and Dunlin, ordered by number of birds lost, with Knot seeing the biggest absolute decline.

In preparing the new estimates for the British Birds paper, an opportunity was taken to refine the way that populations are calculated, based on Use of environmental stratification to derive non-breeding population estimates of dispersed waterbirds in Great Britain, by Verónica Méndez et al. The new methodology explains some of the differences between percentage changes reported by WeBS and the percentage changes obtained by comparing the latest population estimates to those in APEP3.

blog KN graphic

The Knot estimate dropped from 320,000 to 260,000. This figure is higher than might be expected from the counts that take place at sites covered by WeBS, being larger than the ten-year decline of 14% reported in the last WeBS report. Knot are mobile species within the North Sea and Atlantic Coast wintering area and it is possible that British losses may be explained, at least to some extent, by redistribution.

blog oyc graphThe drop in Oystercatcher numbers from 320,000 to 290,000 appears to be less than 10%, compared to a ten-year decline of 12% on WeBS. Improved analysis of NEWS data helped to add some more birds to the open-coast estimate so the 10% fall may underestimate the seriousness of the Oystercatcher situation. The 25-year Oystercatcher decline on WeBS is 26%, which is not surprising if you look at the changes to breeding numbers in Scotland, where most British birds are to be found. There’s more about this in: From shingle beach to roof-top.

blog RKThe Redshank decline of 26,000 is higher than would be predicted from WeBS figures, suggesting a drop of over 20% since APEP3, rather than ‘just’ 15% for the ten-year WeBS figure. This is a species that also features strongly in the Non-estuarine Waterbird Survey and that might explain the difference. Wintering Redshank are mostly of British and Icelandic origin, with the Breeding Bird Survey (BBS) suggesting a ten-year decline of 24% in our British breeding birds.

The Curlew is now globally recognised as near-threatened. The latest winter estimate is 120,000, down from 140,000 in APEP3. The new total represents between 14% and 19% of the European population, which means that we have a particular responsibility for this much-loved species. Only the Netherlands holds more wintering Curlew than Great Britain. Is the Curlew really nearly-threatened? is one of several blogs about Curlew in the WaderTales catalogue at www.wadertales.wordpress/about .

blog 2 DNIt has been suggested that the long-term declines of Grey Plover and Dunlin  may be associated with short-stopping, with new generations of both species wintering closer to their eastern breeding grounds than used to be the case. WeBS results indicate a 31% drop in Grey Plover and a 42% drop in Dunlin, over the last 25 years. There was a loss of 10,000 for both species between APEP3 and the new review, representing declines of 23% and 3% respectively.

The biggest winners

There are four big winners in the period between APEP3 (2004-09) and the new review (2012-16), although, even here, not all as it seems.

The Avocet has seen further dramatic gains. with the estimated wintering population rising to 8,700. The increase is not quite as big as might have been expected, based on the 43% rise seen in ten years of WeBS counts, but it is still a dramatic continuation of a 40-year trend.

The numbers of Bar-tailed Godwit and Ringed Plover are both substantially higher but at least a proportion of each of these changes is linked to the better coverage and more sophisticated sampling methods that were discussed earlier. Bar-tailed Godwit increases may also reflect redistribution around the North Sea.

blog BW graphOne of the consequences of improved statistical techniques, as used this time around, is the apparent decline in the estimated population of Black-tailed Godwit. The new figure of 39,000 is 4,000 smaller than in APEP3, despite the fact that the WeBS graph clearly shows an increase. Interpolation using WeBs figures suggests that the earlier population estimate should have been 31,000, rather than 43,000.

There are other winners too, as you can read in the paper. At the start, I posed the question “how many Greenshank are there in the country during the winter?”.  The answer is 810, representing an increase of 200 since APEP3.

Game species

The estimates for the three wintering waders that are still on the UK quarry list have not changed since APEP3 (published in 2011) as there are no new data available.

Golden Plover: The winter estimate remains as 400,000, as there has been no comprehensive, winter survey since 2006/7. Large numbers of Golden Plover arrive from Scandinavia, Europe and Iceland in the late summer, joining the British birds that choose not to migrate south or west. The GB breeding population is probably less than 50,000 pairs. Most breed in Scotland which has seen a breeding decline of 23% in the period 1995 to 2016 (BBS). Golden Plover is still ‘green listed’.

snipe-headerSnipe (Common): The winter estimate remains as 1,100,000 – a figure that was acknowledged in APEP3 as being less reliable than that of most species. At the same time, the GB breeding population was estimated as 76,000 pairs, indicating at least a 4:1 ratio of foreign to British birds, and that does not take account of the number of British birds that migrate south and west. Snipe are ‘amber listed’ but BBS suggests a recent increase of 26% (1995-2016). There is a WaderTales blog about  Snipe and Jack Snipe.

Woodcock: The winter estimate remains as 1,400,000 – another figure that is not considered to be particularly precise, with much variation between years. The diminishing breeding population is dwarfed by winter numbers, as you can read in this WaderTales blog, with increased attention being given to ways to afford better protection of red-listed, British-breeding birds.

January counts

blog BTThe paper in British Birds also includes a table of January population estimates, to provide data that are comparable to mid-winter counts in other countries. These figures are used in waterbird monitoring for the International Waterbird Census for the African Eurasian Flyway. The main table (and figures mentioned above) are average maximum winter counts (in the period September to March). Black-tailed Godwit is one species that illustrates the difference, with a mean of 30,000 in January and a mean peak count of 39,000. Having moulted in Great Britain, some Black-tailed Godwits move south to France and Portugal in late autumn, returning as early as February. January counts are therefore substantially lower than early-winter and late-winter counts. There is more about the migratory strategy employed by Black-tailed Godwits that winter in southern Europe in Overtaking on Migration.

Looking forward

blog BB coverThe authors have done a tremendous job. They have refined the way that estimates are calculated, they have combined the results from WeBS and NEWS III, and they have delivered population estimates for 25 wader species and many more other species of waterbirds. These population estimates will be used in conservation decision-making until the next set of numbers becomes available. Meanwhile, thousands of birdwatchers will count the birds on their WeBS patches in each winter month, every year. Without them, this paper could not have been written.

Before the next assessment, there will need to be another NEWS survey, to check up on species that use rocky and sandy shore birds, such as Purple Sandpipers, Turnstone and Curlew. Hopefully, there will also be a dedicated survey to assess Lapwing and Golden Plover numbers and perhaps we might find a way to refine the old estimates for Woodcock, Snipe and Jack Snipe.

Paper

Population estimates of wintering waterbirds in Great Britain. Teresa Frost, Graham Austin, Richard Hearn, Stephen McAvoy, Anna Robinson, David Stroud, Ian Woodward and Simon Wotton. Published in British Birds Volume 112. March 2019.

British Birds is a subscription journal. The issue containing this paper can be purchased separately. At some stage, the paper will become free-to-view.

blog flying godwits

 


GFA in Iceland

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.

@grahamfappleton

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Wetland Bird Survey: working for waders

Red-listed Curlews, Scottish Oystercatchers, a boom in Black-tailed Godwits and the need for safe roost sites. Here’s a selection of WaderTales blogs that may appeal to counters who contribute to the UK Wetland Bird Survey (WeBS) and other birdwatchers who like waders/shorebirds.

Blog RINGOS

It’s 70 years since UK birdwatchers started to count waders and waterfowl and there are now over 3000 registered Wetland Bird Survey volunteers.

WeBS70logo6a_smallThe work that volunteers do to chart the rises and falls of species as diverse as Redshanks and Whooper Swans provides a unique insight into the fortunes of our wintering waterbirds. As a tribute to the people behind the binoculars and telescopes, I highlight seven WaderTales articles that use WeBS data. Click on the links in bold if you want to read a particular story.

Curlew counts

curlew

WeBS counts for Curlew in Great Britain between 1974 and 2016

In the blog Is the Curlew really near-threatened? WeBS counts are used to show how numbers have changed over the decades. There might have been a boost in numbers when Curlew came off the hunting quarry list in Great Britain in 1981 but declines in the last 15 years reflect issues birds face in the breeding season in many parts of their European range.

Internationally, Eurasian Curlews are classified as near-threatened and in the UK they are now red listed. WeBS counts in Northern Ireland, alongside I-WeBS counts in the Republic, were successfully used to argue for the cessation of shooting across the island of Ireland in 2012.

Scottish Oystercatchers

L17A9623 (2)

Oystercatchers are unusual, amongst waders, in that they feed their young

Surely the Oystercatcher is one wader species that we don’t need to worry about? Although the blog Oystercatcher: from shingle beach to roof-top leads with nesting behaviour, WeBS counts are used to illustrate regional trends in different parts of the United Kingdom. In Scotland, there is concern about poor breeding success, while in parts of Wales and England, WeBS counts may provide a way of measuring the population-level effects of cockle fishing and diseases affecting shellfish.

oyc webs

Three very different trajectories for national WeBS counts for Oystercatchers since 1974

Mid-winter movements

figureThe annual WeBS report highlights the months in which counts are at their highest in different estuaries. For Knot, for instance, the highest counts on the Wash are in September, in other east-coast estuaries and on the Dee the peak is in December, whilst further north, in Morecambe Bay and the Solway, top numbers occur in January and February.

In Godwits & Godwiteers, which focuses on the superb work of observers who track the movements of colour-ringed Black-tailed Godwits, WeBS counts from east coast estuaries and the Ouse Washes illustrate the move inland that occurs as the winter progresses.

National patterns and local counts

blogGroups of WeBS counters who cover local estuaries will be the first to notice changes in the numbers of the key species that use their own sites. If the number of Dunlin drops, is that a local phenomenon or part of a national picture? Is there always a strong link between national declines (or increases) and site-based counts? Interpreting changing wader counts provides some answers. It emphasises just how reluctant waders are to change wintering sites between years.

High-tide roosts

horse-and-flockEvery WeBS counter will appreciate the value of a safe (undisturbed) roosting site, whether this be used by waders or by ducks and geese. In A place to roost, WeBS counts for Black-tailed Godwits are used to assess the national and international importance of an individual roosting site in northwest England. The main thread, however, is about the energy expenditure associated with sleeping (not very much) and travelling to and from a safe roost site (lots). An interesting add-on is the story of what happened to Cardiff’s Redshanks when the estuary was turned into a lake.

New recruits

If adult birds don’t change their winter homes then increases in local populations may well reflect good breeding years for wader species. 2017 was a good year for several species that breed in Iceland, particularly Black-tailed Godwits. T with BTGA great summer for Iceland’s waders puts the year’s productivity into context and gives an update on wader research that is being undertaken by the South Iceland Research Centre (University of Iceland), the University of East Anglia (UK) and the University of Aveiro (Portugal). If you have ever seen a colour-ringed Black-tailed Godwit, Ringed Plover, Oystercatcher or Whimbrel you may well find this interesting.

On the open shore

NEWS tableThe blog News & Oystercatchers was written to promote the Non-estuarine Waterbird Survey of 2015/16, or NEWS-III. There are a lot of waders on the shorelines that link the estuaries that are covered for WeBS and, every few years, volunteers are asked to count these birds. In NEWS-II (2006/07), it was estimated that 87% of Purple Sandpipers were to be found on the open shore (see table) with high numbers of several other species. There’s an initial assessment of the results for NEWS-III in the latest WeBS report and I look forward to writing up the results as a WaderTales blog, once a paper is published.

Links to blogs mentioned already

Many more to choose from

There are over 40 WaderTales blogs to choose from in this list. Four of these articles might be of particular interest to WeBS counters:

  • knot

    Knot migration

    Which wader, when and why? gives an overview of the migration of waders into, out of and through Britain & Ireland. The patterns help to explain why the peak numbers for Sanderling occur on the Wash in August, on the Dee in November and on the North Norfolk coast in May, for instance.

  • Bar-tailed Godwits: migration & survival  contrasts the different migration patterns of the two races of Bar-tailed Godwits that use British & Irish estuaries and explains the importance of colour-rings in the calculation of survival rates. On the other side of the world, Wader declines in the shrinking Yellow Sea shows how quickly numbers can change if the annual survival probabilities of adults fall. sum plum
  • The not-so-Grey Plover focuses on the Grey or Black-bellied Plover but the real story is about moult. British and Irish estuaries are important to huge numbers of moulting waders. WeBS counters often don’t have time to look at individual birds but, with the right camera, you can learn a lot about waders by checking out the right feathers.

Thank you

Blog Counter 1I use WeBS data a lot – in my blogs and in articles – and I appreciate the tremendous value of data collected each month by thousands of contributors. They monitor the condition of their local patches and have directly contributed to local, national and international reviews of the conservation status of wintering waterbirds. To every current and past WeBS counter – ‘thank you!’

There’s a (large) selection of papers using WeBS data here, on the BTO website. The Wetland Bird Survey is run by the BTO, in partnership with RSPB and JNCC (which acts on behalf of NE, NRW, SNH & DAERA), and in association with WWT.


 GFA in Iceland

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.

@grahamfappleton

The not-so-Grey Plover

sum plumThe Grey Plover may be grey in the winter but in its summer plumage the American name of Black-bellied Plover is far more appropriate. Each spring, adults moult into this breeding finery but what happens to first-years, are there differences between the sexes and are Grey Plovers ‘typical’ of the wader family as a whole?

This blog is inspired by the moult chapter from Shorebirds in Action, a new book by Richard Chandler, published by Whittles Publishing. The descriptor “an introduction to waders and their behaviour” gives a clue to the fact that Richard has used his excellent photographs to illustrate aspects of shorebirds’ lives, such as moult, migration, territorial behaviour and physiology. Many of the photographs in this blog have been provided by Richard.

‘Dressing for the Occasion’

Back in the ‘old days’ a group of wader ringers would huddle over the outstretched wing of a shorebird, often in the dark, by the light of a paraffin lamp, in order to try to understand the moult of individual birds. In the case of the Grey (or Black-bellied) Plover, the primary feathers sometimes revealed an interesting story of previous difficulties experienced by an individual. More of this later.

Thanks to the development of digital photography and long lenses, the study of moult is no longer restricted to ringers looking at birds in the hand. Birdwatchers can all share in the detective work and learn more about how plumages of waders change with age and season, allowing us to identify males and females and to think about the stresses that individuals face.

The moult of Grey Plovers

Most waders moult their main flight feathers in the autumn and take on summer plumage in the spring. This adult Grey Plover (below) was photographed on the Wash in Norfolk in August. It is not in full summer plumage, having started its moult before migrating from the Taymyr region of Siberia. There’s lots more about wader migration here. Two of the primaries (P1 and P2) are new, with crisp white edges to the feathers. Over the course of the next few weeks, this bird will recommence moult by dropping P3, with moult then progressing outwards through the primaries and inwards through the secondaries. The main feather tracts are labelled.

structure of wing

Summer plumage feathers can be seen in the scapulars and in the coverts. In some individual Grey Plovers, and in other species, there can also be summer plumage in the tertials. In this photograph, these are hidden by the large, showy scapulars. Juvenile birds of many species are often aged by looking for buffy tips on the median coverts. The inner median coverts are protected from wear by the scapulars, so this is the best place to look for hidden evidence that a bird is a youngster. This is where to find the odd buffy fringe in a young Dunlin in its first spring, for instance, when it is  about 10 months old.

adult winterFor an adult wader, the post-breeding moult is complete. The photograph alongside shows an adult bird in full winter-plumage bird; the picture is from Norfolk (UK) but the same photograph could have been taken in South Africa, India, Australia or South America, so global is the Grey Plover’s distribution. The bird has discarded all of the breeding plumage it acquired in the spring, and the flight feathers that were a year old. All of the coverts of the wing have been moulted and have a uniform appearance, and look at the fresh-edged tertials that fold neatly over the dark (almost black) tips of the primaries.

juvenileThe next photograph shows a first-winter juvenile, newly arrived from the breeding grounds. This bird is a Black-bellied Plover in Florida. Having started life as a tiny, downy chick, it will have acquired this new set of feathers before migration. The spangling of the back is diagnostic of a juvenile Grey Plover but the golden spots can confuse a novice wader-handler, who might want to turn it into one of the golden plover family. Before winter starts, new non-breeding-type body feathers moult in, so the bird loses the spangling of the upper parts and a lot of the streaking of the under-parts.

first winterIt is not thought that one-year-old Grey Plovers breed in their first summer, with most staying on their ‘wintering’ grounds for well over a year, through to the second April or May. The photograph alongside was taken in May in Norfolk; the bird may have been sharing the beach with gorgeous summer-plumage birds that were preparing to depart for Siberia. Grey (or Black-Bellied) Plovers are nearly circumpolar breeders. They are missing from Greenland and Northern Scandinavia but breed across the northern coasts of Russia, Alaska and Canada – only three countries but a vast area of tundra.

age characteristicsThe next figure is a composite of two of the photographs, created to show the key diagnostic features of an adult and immature in winter plumage. The notches on the tertials are particularly pronounced in the young bird because the feathers were already nearly a year old at this point – it is not always this easy! When assessing the age of any wader, the tertials and coverts are key feathers to check out.

Primary exceptions

In many ways, the Grey Plover is a fairly typical medium-sized wader; adults moult their main flight feathers (wings and tail) in the autumn but juveniles have to wait until the early summer of their second year before they do the same. This first primary moult of these young birds may start as early as June and is completed in August or early September, two months ahead of breeding adults. To learn more about Grey Plover moult see this paper by Nick Branson & Clive Minton from the Wash Wader Ringing Group.

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Smaller waders, such as the alpina Dunlin that spend the winter in Britain & Ireland, typically breed when only a year old. Unlike Grey Plovers, Dunlin moult into breeding plumage in their first spring. When they return from Russia in the same summer they can still be aged if there are juvenile inner median coverts with unworn buffy fringes. Primaries in these one-year-old birds tend to be more worn than those of adults, being three months older and having been used for one more migratory journey.

In some species, such as Semipalmated Sandpiper, you can find young birds in the spring with a small number of fresh outer primaries. By moulting the outer, and most worn, of their flight feathers during the wintering period they are presumably able to increase flight efficiency for the journeys north and south. Young birds of the tundrae race of the Ringed Plover take this early primary renewal strategy even further, by undergoing a complete primary moult in their first winter, while on the wintering grounds.

Under stress        

Moult is an energetically expensive activity. When resources are low, it may not be possible to complete the moult process. In this BTO report by Phil Atkinson and colleagues, about wader mortality associated with die-offs of cockles and mussels in the Wash (UK), they showed that more Oystercatchers suspend moult in years with very low food supplies. In these ‘bad shellfish years’, it was not uncommon to find Oystercatchers with one or more unmoulted outer primaries – old feathers that would be retained until the next autumn moult.

suspendedThe same thing can happen in Grey Plovers but it is more likely that they will suspend moult in the autumn and then finish in March. In the Branson & Minton paper mentioned above, it was estimated that 30% of adults suspended moult for the harsh winter months. The paper was written in 1976 and it would be interesting to know whether the proportion of birds suspending has dropped, given that winter weather now tends to be less severe.

Suspended moult is found in Grey and Black-bellied Plovers across the world. The photograph alongside was taken in Florida on 20 December. The unmoulted outer primary is more pointed and browner than the other nine.

Gender

greplThe gender of winter-plumage Grey Plovers cannot be determined, even in the hand, but the summer plumage is pretty diagnostic. A bird with a jet-black belly, with perhaps the odd white tip to a feather, and an almost pure white band behind the black face and neck is probably a male (as here). If the bird is less well-marked – brownish-black on the front with more mottling – then you are likely to be looking at a breeding plumage female. Come the autumn, all of this finery will disappear and the Black-bellied Plover will become a Grey Plover once more.

Moult in other species

cover with outlineThis blog focuses on the moult of one species. If you want to learn more about moult in general and many other aspects of the behaviour of waders then immerse yourself in Shorebirds in Action, An Introduction to Waders and their Behaviour by Richard Chandler. Click here for link

Moult features strongly in this WaderTales blog about Black-tailed Godwits and in this blog about Lapwings.


 GFA in Iceland

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.

@grahamfappleton