Will head-starting work for Curlew?

83 captive-reared Curlew were released successfully in 2019, over 130 in 2021 and a similar number in 2022 but this does not mean that head-starting is a solution to England’s Curlew problems. We don’t yet know the proportion of youngsters that survive the difficult ‘teenage years’, how many will find suitable breeding habitat and whether these birds can reliably raise chicks of their own. What have birds such as 7K, pictured here, revealed and what might happen next?

Exciting news

There has been some great TV and press coverage of head-started Curlew this summer (2022), with films of captive-reared birds being released from pens and flying for the first time. But what does the future hold for naïve birds that take to the air in Shropshire & Powys, the Severn & Avon Valleys of western England, Cornwall, Sussex and Norfolk? The first signs are good, as illustrated below in maps published by the BTO for the birds that they are tracking in Norfolk.

Satellite-tagged birds released at Ken Hill (Black stars – close to the Norfolk coast) can be seen flipping backwards and forwards between the mudflats and their release site, before beginning to explore more of the Wash estuary.

Birds released further inland, on The Sandringham Estate (White stars), started foraging in grassland areas and it took a while until any of these birds discover the coast. One bird “7Y” moved onto a little-used airfield (below), where it was seen with a flock of adults. It appears to have adopted a mudflat/grassland tidal routine, involving ‘commutes’ of ten miles each way, possibly having been guided by the adults (top row, third from the left).

One of the Sandingrham youngsters joined up with a flock of adults on an airfield five miles from the release site

It is easy to get ‘wowed’ by the TV reports and the individual stories of tracked birds but these are early days for Curlew head-starting. There’s a lot still to learn!

Why head-start Curlew chicks?

This Curlew is nearly ready to fledge

The Eurasian Curlew is in trouble, as you can read in the WaderTales blog “Is the Curlew really near-threatened?”. Annual survival rates of adults are high but productivity is low, with an estimate of 10,000 too few youngsters being fledged each year, to maintain population levels in the UK. The Irish Curlew population has crashed, providing dire warnings of what may happen soon in Wales, where there has been a decline of 73% since 1995 (Breeding Bird Survey). In southern England, most of the losses happened before 1995 and only a few populations remain, hanging on in areas such as Breckland in East Anglia.

Lessons from Black-tailed Godwits

We know that head-starting works well for limosa Black-tailed Godwits breeding in East Anglia but there was no guarantee that this would be the case when the WaderTales blog Special Black-tailed Godwits was published in 2017, to coincide with the departure of captive-reared birds from their WWT Welney release site. When the first birds not only returned in 2018 but actually raised their first chicks (see Head-starting Success), the approach looked to be successful. Over the following years, with more releases, the godwit population in the Ouse and Nene Washes has increased significantly. This temporary relief package for a beleaguered population has worked well, providing time to try to improve breeding conditions in the wild, so that Black-tailed Godwits can look after themselves. Head-starting is an intervention of last resort and cannot be a long-term solution to the problems faced by the species.

Young Curlew is now identified as 2X

It is possible to support local populations of scarce birds, such as Bitterns and Little Terns, with interventions such as increasing areas of suitable habitat and protecting nest sites, but doing the same for Curlew will be more of a problem. Here, the challenge is to halt a decline of a species that breeds across wide areas of the UK, mostly on farmed land that is not being managed for conservation. Intensive, local action is not going to be enough. This is going to need teamwork between landowners, conservation organisations and volunteers – acting at a scale that has not been tried previously. Perhaps head-starting will help?

A pragmatic plan

The most recent UK population assessment of Curlew, in British Birds, suggested that there were 58,500 breeding pairs in 2016. Numbers have almost certainly dropped since then but the species is nowhere near as threatened as England’s breeding Black-tailed Godwits. The biggest Curlew head-starting programme, led and funded by Natural England, was a response to an opportunity to use unwanted Curlew eggs – not a ‘last-chance’ solution, as it had been for Black-tailed Godwits. Each year, Curlew eggs were being collected on RAF airfields, under licence, to deter adults and reduce airstrike risk. Thanks in no small part to Natural England’s Graham Irving, many of these eggs are now being head-started, instead of destroyed.

What happens to head-started birds?

Curlew eggs in an incubator

The Natural England project builds upon the experiences of the Curlew Country team, working in the Shropshire hills and Powys borders. They released 6 head-started Curlew in 2017, 21 in 2018, 33 in 2019 and 34 in 2021. Raising extra chicks is part of an initiative that involves a broad range of stakeholders. See their website for more information.

The first stage of the head-starting process works well. Aviculturalists can rear and release chicks, with very high rates of success. They are learning more and more about the best diets, appropriate husbandry and the release process. Individual chicks wear small colour-rings, so that progress can be monitored daily, and they receive leg-flags and get weighed and measured before release. For instance, the young Curlew 7K featured above is a GPS tagged male that weighed in at a respectable 550 grammes when it was transferred to its release site on The Sandringham Estate, on 14 July 2022. By the end of August, it had moved to the Wash Estuary, as you can see in the figure above (top left map).

Before transfer to The Sandringham Estate release site, a young Curlew is checked over by a vet

From the start, the Natural England project has been built on partnerships. In the first year (2019), eggs were hatched and chicks were reared at WWT Slimbridge. Fifty young Curlew were released near-by and the first of these birds (wearing ring number 23) was found nesting near Gloucester in 2021, raising one chick of his own. In 2022, three years after release, five of the head-started Curlew were on breeding sites in the Severn and Avon area and one bird had moved to the Thames Vale. Male “23” shows that head-starting can add more Curlews to the breeding population but is he exceptional? What proportion of released birds make it this far and go on to raise chicks? The video by Kane Brides in this WWT blog tells the West Country story so far.

After a Covid-caused hiatus in 2020, the head-starting project expanded in 2021, with birds reared at both WWT Slimbridge in Gloucestershire and Pensthorpe in Norfolk. Richard Saunders, the Senior Ornithologist for Natural England, recognised the importance of learning what happens to newly-released fledglings and of monitoring potential recruitment. Working with Sam Franks of BTO and with lots of support from others, the Pensthorpe-reared Curlews are starting to reveal some of their secrets.

Where will the Pensthorpe birds set up home?

Most waders are philopatric; when they look for breeding sites, they tend to return to places close to the places in which they were raised. The release sites in North Norfolk were chosen not only because they provided suitable conditions for growing teenage waders, with low predator numbers and adults feeding nearby, but also because they are relatively close to current breeding sites, such as Roydon Common and the heaths of Breckland (see Curlews and foxes in East Anglia). There are also airfields, of course, and it will be unfortunate if birds choose these sites, given that eggs were removed to try to reduce bird-strike risk.

Radio tag (left) will drop off when juvenile feathers are replaced. Satellite tag is mounted on a temporary harness and worn like a rucsac.

All of the chicks are colour-ringed, some of them are radio-tagged and others are GPS-tagged, prior to release. By following tagged birds, as they explore the area around the release sites, the project aims to understand more about habitat use and to see if these naïve birds seem unduly prone to predation, given that they have not been trained to look out for danger by watchful parents. GPS tagging helps to paint the bigger picture; would birds move to the mudflats of the Wash and spend the whole winter in Norfolk or would some move on, to southern England, Ireland or France, for instance? These data will hopefully be augmented by reports of colour-ringed birds that do not carry tags.

2C takes to the air after release at Ken Hill

The first results have been encouraging. For example, a GPS tagged female from the first cohort released in Norfolk in 2021, wearing flag 0E, has followed the ‘stay local’ option. She spent the winter on the saltmarshes of RSPB Frampton, on the Wash in Lincolnshire, and has largely stayed on the south shore of the Wash through her first summer, showing no evidence of visiting any type of breeding habitat yet. A male Curlew wearing flag 4P has been more adventurous, spending the winter on the Exe Estuary in Devon, a site used by some adult Curlews that breed in Eastern England.

The amazing left-hand map below shows the route taken by ‘6Y’, one of the first batch of 2022 birds to be released on The Sandringham Estate. The story of this bird is told by the BTO’s Dr Sam Franks:

“The first of this year’s birds to migrate away from Norfolk departed at sunset last Wednesday & arrived on a Staffordshire field at sunrise on Thursday. It then flew towards Ireland & made an anxiety-inducing trip out into the Atlantic before returning to dry land.”

The right-hand map shows an overnight flight by ‘9L’, one of the last birds to be released from Ken Hill. It set off on the evening of 16th September, flew southwest overnight and headed south when it ran out of land. It landed in France at 01.30 on 17th.

Data generated by colour-ring sightings will be analysed to check whether annual survival of youngsters in their first couple of years are consistent with figures for wild-reared chicks. This follow-up work is really important – the head-starting operation may seem to be successful but if few chicks survive long enough to breed then alternative approaches may be needed. Sightings of colour-ringed birds provide important information to add to data collected from tagged birds, which means that there is a vital role to be played by birdwatchers.

Around England

People care deeply about Curlew, as Mary Colwell explored in a recent book, reviewed in the WaderTales blog Curlew Moon, so it is not surprising that landowners feel inspired to help the species, by releasing birds on their own land. An attempt is being made to boost a tiny population on Duchy of Cornwall land on Dartmoor, using eggs from East Anglian airfields that were hatched at Slimbridge, and a licence has been granted to take eggs from a site in Northern England to be reared in Sussex, on an estate that appears suitable but where Curlew do not currently breed.

Release sites for these translocated birds do not hold many (or even any) Curlew so it will be interesting to learn whether the behaviour patterns of these fledglings are different to those of birds released in Norfolk and in the Severn and Avon Valleys of western England. All birds are being ringed, by WWT, GWCT and BTO ringers, with every report of a marked bird adding to our understanding of the success of the various projects. In each case, hopefully there will be enough money to deploy dedicated fieldworkers to monitor what happens to the released birds during the crucial first few weeks of independence, so that fledging rates can be accurately assessed, and to monitor return rates in subsequent breeding seasons. For slow-maturing birds, this follow-up work will involve a five-year commitment.

Maximising effectiveness

The work being funded by Natural England in East Anglia is expensive. It will be judged as successful if released birds augment local populations, whether these be in East Anglia (as hoped) or in another part of lowland England. If birds choose to breed in the North of England, where numbers are still high, or even overseas, then that may make it difficult to justify the expense of further rearing and monitoring work.

There is a concern that head-starting will be seen as a solution to the problems being faced by Curlew. It isn’t! The estimated shortfall in fledged chicks is 10,000 birds per year, across the whole of the UK, and head-starting will never make a big impact on that number. It may be a way to boost numbers in lowland areas, from which the species would otherwise be lost, but only if the conditions for successful breeding can be created and maintained. This means tackling the thorny problems of habitat degradation and predator numbers. Head-starting may seem like a dynamic intervention but if birds are released into areas where breeding success is too low then it’s not going to produce a sustainable solution to the problems being faced by England’s Curlew.

Photos of ringed birds are particularly appreciated

The Norfolk head-starting project would appear to tick all the right boxes – release sites with low predator numbers, right next to an estuary with lots of Curlew and with successful breeding sites near-by. Head-started birds soon start to explore the mixture of arable and tidal resources available to them and it looks as if early-years survival rates might be as high as expected of wild-reared birds. Despite all these positive signs, it will be a couple of years until this year’s young Curlew start breeding – somewhere – and that will be the crucial test of head-starting. Meanwhile, it is hoped that birdwatchers will look out for colour-ringed birds, so that survival rates can continue to be monitored. Every Curlew counts!

Norfolk-ringed head-started Curlew wear yellow flags, each with number & letter, immediately above an orange ring. Please report sightings of colour-ringed head-started Curlew using THIS LINK.

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.

When mates behave differently

Iceland’s 40,000 Oystercatchers are an interesting mix of resident birds and migrants, providing an ideal system in which to study the costs and benefits of the two options, and to try to work out what influences whether an individual becomes a ‘resident’ or a ‘migrant’. I’ve added the inverted commas because many residents migrate within Iceland in spring and autumn; it’s just that they don’t undertake long-distance flights across the Atlantic.

In their paper in Ecology & Evolution, Verónica Méndez and colleagues from the South Iceland Research Centre (University of Iceland), the University of Aveiro (Portugal) and the University of East Anglia (UK) investigate the timing and success of breeding attempts by resident, migratory and mixed (resident/migratory) pairs of Icelandic Oystercatchers.

Iceland’s Oystercatchers

As outlined in Mission Impossible: counting Iceland’s wintering Oystercatchers, about 30% of Icelandic Oystercatchers never leave the country, coping with cold temperatures, short December and January days and a restricted diet. In the winter months they can be found in the tidal zone of a few estuaries, mostly in the warmer west.

The majority of Iceland’s Oystercatchers fly 1000 km or more across the Atlantic, to Ireland, the UK and the coastal fringe of western Europe. Here, many colour-ringed birds have been spotted by birdwatchers, who play a vital part in migration studies. The blog Which Icelandic Oystercatchers cross the Atlantic? revealed that males and females were equally likely to migrate, while there appeared to be no assortative mating in spring (residents don’t preferentially choose resident partners, for instance).

It would be easy to envisage circumstances in which resident birds might be at an advantage, at the start of the spring breeding season, having not had to cross the Atlantic and thus being ready if an early nesting opportunity opens up. On the other hand, in a cold spring or after a particularly harsh winter, perhaps they could be in poorer condition than newly arrived migrants, and this may potentially delay breeding. What actually happens?


Both resident and migrant Oystercatchers breed throughout lowland Iceland. Within breeding pairs, it is estimated that about 20% of pairs are resident, 46% are migrant and 34% are mixed. These are long-lived birds that generally maintain the same partners between years, despite the fact that individual males and females may spend seven months of the year up to 3000 km apart. Parents tend to be equally involved in incubation duties, territorial defence and chick rearing, although males tend to remain with their youngsters longer than do females.

Between 2015 and 2018, Verónica Méndez and her colleagues monitored the breeding attempts of Oystercatchers in southern Iceland, continuing a study of marked individuals that started in 2013. Adults were caught on the nest and sexed by later analysis of feather samples. With the help of a network of volunteer observers, the winter locations of 186 (out of 537) marked birds had been established when the paper was first written. Using these known outcomes and with additional information from stable isotope analysis, it was possible to assign the remaining 351 birds as ‘residents’ or ‘migrants’. Amazingly, 73 of these 351 birds have been seen since the isotope data were analysed and all of the assumptions on winter locations were found to have been correct.

Early nesting attempts may be hampered by spring snowfall

The first migrant Oystercatchers arrive in Iceland in February but no nesting has been recorded before mid-April. Searches for colour-ringed birds and nests were conducted every 2-3 days and then nests were followed through to hatching or failure. Second (and third) nesting attempts were also monitored. Oystercatchers remain in the vicinity of the nest after hatching their chicks and then feed them throughout the growing period. Chicks were metal-ringed just after hatching and individually marked with colour-rings when around two weeks old. Families were monitored every 3-4 days until all chicks were fledged or lost, allowing productivity (number of chicks fledged per pair) and fledging success (number of chicks fledged in nests where at least one egg hatched) to be recorded.

Who breeds when?

Verónica and her colleagues were able to estimate laying dates for 138 pairs with known migratory behaviour (56 migrant, 50 mixed and 32 resident pairs) in one or more seasons during 2015-2018, providing a total of 228 observations.

The top graph shows that, on average, 2015 was a much later breeding year than the other three. This was a colder spring; the sort of colder conditions that an older Oystercatcher may well have encountered frequently in its youth! (The longevity record for BTO-ringed Oystercatcher is 41 years – see Waders are long-lived birds – and the trend for there to be more frequent warmer springs is discussed in this Black-tailed Godwit blog).

The lower graph shows a breakdown of the data into the three categories – Resident (black dots), Mixed (grey) and Migrant (white). There is no difference between the egg-laying dates for residents across the four years. However, in the 2015 breeding season, in cases where either member of the pair is a migrant, there was an average nesting delay of over a week. An analysis in the paper shows that it does not matter which member of a mixed pair was the migrant, the delay in 2015 was the same.

Reproductive performance

Unusually amongst waders, adult Oystercatchers feed their chicks

As expected, Oystercatcher pairs that made earlier nesting attempts were more likely to lay a replacement clutch after nest loss, had higher productivity and higher fledging success. This is in line with the modelling paper described in Time to nest again. Early-nesters tended to have bigger clutches too. Any differences between the performance of residents, mixed pairs and migrants could be accounted for just by the timing of nest initiation.

In the papers’ Discussion, the authors suggest that, in the three warmer years, earlier nesting of pairs that included at least one migrant was sufficient to slightly enhance nest success but not overall productivity, above that achieved by pairs with residents. The migratory behaviour of the male within a pair appeared to have a stronger effect on fledging success than the migratory behaviour of the female, suggesting that males may play a more important role than females at the chick stage. This is interesting in the context of previously-published research by Verónica and her colleagues, as described in The Dad Effect blog.

What does this all mean?

In other studies, described in the Discussion, residents in systems where some individuals migrate have been found to have advantages over migrants, because they can get on with breeding earlier. This was not the case for Icelandic Oystercatchers, potentially because migrants can arrive in good condition in all but the coldest of years.

Hatching brood of three

In the cold year of 2015, Oystercatcher pairs nested an average of between a week and 12 days later than in other years. This delayed nesting occurred in migrant and mixed pairs but not in resident pairs, suggesting that the effect of the severe weather may have been greater on migrants than residents. Cold spring conditions in Iceland tend to be part of a wider pattern of cold weather across northwest Europe. The authors suggest that wintering conditions might influence the body condition required to reproduce and that these conditions may be more variable for migrants.

Only one cold year occurred during this study, so the authors don’t know whether pairs with migrants consistently breed later in colder years. Given that cold springs are increasingly rare in Iceland, 2015 may turn out to have been one of the few remaining opportunities to reveal the dynamic nature of links between weather, migratory behaviour and breeding phenology at these latitudes.

One potential explanation of the difference in the timing of nesting is the effect of habitat. The Icelandic team has found that there is a strong tendency for migrants to breed inland, whereas residents tend to breed along the coast. During the cold spring of 2015, inland habitats were not available as early as in the following years (everything was frozen), mostly delaying the breeding attempts of migrant and mixed pairs, rather than residents pairs.

Long-term studies

Verónica Méndez with one of the marked birds

The take-home message of the paper by Verónica Méndez and her colleagues is that it pays to nest early, which is not unexpected. Perhaps it is surprising that, in the cold spring of 2015, mixed pairs still bred at the same time as pairs of migrants, suggesting that residents waited for their migrant partners. Perhaps, the benefits of nesting with the same partner are very strong, or finding an alternative mate is difficult or both?

The study suggests that the links between individual migratory behaviour and reproductive success can vary over time and, to a much lesser extent, with mate migratory behaviour. Understanding these effects of pair phenology on breeding success may help researchers to understand the potential impacts of changing environmental conditions on migratory species. Such variation is very difficult to capture unless long-term funding is available. Four years may seem like a long time to observe the same Oystercatchers but, for birds that may easily live twenty years, this is nothing!

The full paper can be found here:

Effects of pair migratory behaviour on breeding phenology and success in a partially migratory shorebird population. Méndez V., Alves J.A., Gill, J.A., Þórisson, B., Carneiro, C., Pálsdóttir, A.E., Vignisson, S.R. and Gunnarsson, T.G. Ecology & Evolution

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.

January to June 2022

Here are brief summaries of the first nine WaderTales blogs of 2022. As ever, I am grateful to the authors of the papers that underpin the blogs; they have worked with me to make sure that I get the stories right! I have not covered every new paper; perhaps your favourite is in the pipeline or perhaps I did not happen to spot it? The blogs are described in order of publication.

Welsh Oystercatchers

To explain how flexible Oystercatchers can be in response to changes in their food supply, Katharine Bowgen has brought together long-term data collected by wader ringers and WeBS counters, and added in annual assessments of cockle stocks on the Burry Inlet (South Wales). This paper has a particular resonance, as I remember teaching students about the Burry Inlet Oystercatcher controversy of the 1970s, when complaints from shellfishers led to the deaths of thousands of birds. We understand more about the relationship between shellfish stocks and bird numbers now but what happens when Oystercatchers can’t find food? This paper makes a strong case for the protection of networks of sites, so that individuals have alternatives when needed.

Australian stock-take

One of the great joys of writing WaderTales blogs is that I get to ‘visit’ the flyways of the world without having to burn carbon. How many shorebirds use the East Asian-Australasian Flyway? is a flyway-wide stock-take of the waders that visit Australia and New Zealand, led by Birgita Hansen. It is shocking that a flock of 350 Far-eastern Curlew now constitutes 1% of the global population and that the population of Curlew Sandpipers has halved in double-quick time, but the key strength of the paper is the clear explanation of a methodology that can be used in the future, to monitor changes in numbers.

Chick vocalisation

Big analyses of data sets are very important but it’s lovely when you learn more about the natural history of species that birdwatchers know well. In Australia, Kristal Kostoglou recorded the calls of the chicks of Red-capped Plovers and Southern Masked Lapwings, that were being ringed and measured in the hand. In Chick squeaks I describe how calls get deeper with age, which is not surprising, but that the calls of males and females can become distinguishable from a very early age. Male Red-capped Plover chicks are more demanding than their sisters!

Trans-oceanic migration

There have been several recent wader papers that interpret data obtained from birds when on migration. One of the interesting questions being asked is, “Do shorebirds account for wind displacement continuously or correct for drift later?”. Navigating a vast ocean summarises Jenny Linscott’s work on Hudsonian Godwits, as they cross the Pacific Ocean and then the Gulf of Mexico, on their way from Chile to Alaska. She and her fellow authors show that flocks make continuous adjustments, demonstrating that birds ‘know where they are’ and giving them the ability to fly extremely long distances without running out of energy. There’s some clever maths too!

Hiding in the trees

In the second paper from her PhD, Triin Kaasiku looks at the breeding success of Estonian coastal waders that nest at different distances from woodland. Keep away from the trees describes these ‘edge effects’. In a part of the world where waders are in diminishingly short supply, hatching success is six time as high in open areas as in areas that are within one kilometre of forest edge. The Baltic coast used to be a haven for species such as Curlew and Dunlin but reduced grazing and forestry plantations have provided hiding places for predators. Alongside increased predation, breeding waders are also having to contend with an increasing numbers of nest inundations, arising from summer storms.

Curlew hunting

Curlew hunting stopped in Great Britain in 1982, when the declining wintering population received protection under the new Wildlife & Countryside Act. A fascinating paper by Ian Woodward and BTO colleagues teases apart the positive effects of the cessation of shooting and more benign winter weather. It is summarised as Curlew: after the hunting stopped.

I am old enough to remember when Curlew were hunted in East Anglia. The pâté made from autumn-shot birds is reputed to have been very tasty; I recall Clive Minton getting back in his land-rover and reporting that he had been offered some, when asking for permission to cannon-net Curlew on a Norfolk land-owner’s estate.

Personal appreciation of Whimbrel

On 27 April, Jenny Gill and I were at Eyrarbakki, on the south coast of Iceland. As we watched, small groups of Whimbrel were coming in off the sea. Others were resting on the seaweed-covered rocks, a few were feeding and some flew straight by. Watching waders arrive in Iceland is always magical but, from sightings of satellite-tagged Whimbrel, we could be pretty sure that these tired birds had just completed five-day, direct flights from west Africa. I could not wait to get back to base and to share our observations. It was a good excuse to round up the Whimbrel stories in other WaderTales blogs, as you can read in Whimbrels arrive in Iceland.

Power-line problems

We have seen huge changes in Iceland, since we first visited in 2000, but how are these affecting shorebirds? In the first paper of her PhD (Effects of land conversion in sub-arctic landscapes on densities of ground-nesting birds), Aldís Pálsdóttir investigated how distributions of breeding waders are affected by power-lines. She discovered significantly depressed numbers several hundred metres from the transmission lines, with Whimbrel and Redshank being the most obviously impacted. Her results are written up as Power-lines and breeding waders. With an increasing global reliance on electricity, these are important findings for planners and conservationists.

Conflict with forestry

Just a few days later, Aldís Pálsdóttir’s second paper was published. By mapping distributions of breeding waders in the vicinity of forests, she has shown that new plantations have a massive effect on distributions. In lowland Iceland, the most vulnerable species appear to be Dunlin and Oystercatcher, followed by Whimbrel, Black-tailed Godwit and Golden Plover. It should be noted that three-questers of Europe’s Whimbrel nest in Iceland, as well as half of the Golden Plover and Dunlin. Aldís and her fellow authors argue that Iceland’s waders need a strategic forestry plan. They estimate that recently-planted woodland and forests have already removed the breeding territories of tens of thousands of waders.

Blogs from previous years

WaderTales blogs in 2021

WaderTales blogs in 2020

WaderTales blogs in 2019

WaderTales blogs in 2018

WaderTales blogs in 2017

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.

Iceland’s waders need a strategic forestry plan

More and more trees are being planted in lowland Iceland – and further increases are planned, in part encouraged by the suggestion that this will mitigate for climate change. Forestry is potentially bad news for Whimbrel, Black-tailed Godwit and other waders that breed in open habitats, and which migrate south to Europe and Africa each autumn. Are there ways to accommodate trees while reducing the damage to internationally important populations of waders?

Pressure on Iceland’s breeding waders

Iceland is changing; more people want second homes in the countryside, the road network is being developed to cope with more and more tourists, new infrastructure is needed to distribute electricity, agriculture is becoming more intensive and there is a push to plant lots more trees. The south of the country is seeing the most rapid loss of open spaces, providing opportunities to study how these incursions affect ground-nesting species, particularly breeding waders.

One of the big changes, especially in Southern Iceland, has been the planting of non-native trees, as shelter belts around fields and country cottages and, more significantly, as commercial crops. Iceland has been largely treeless for hundreds of years but climatic amelioration has facilitated rapid forestry development in areas where tree growth was previously limited by harsher environmental conditions. Seeds of some non-native species are blown on the wind for a kilometre or more, to germinate in open land, well beyond the edge of planned forests.

Most of the new forests are in lowland areas, where we also find the most important habitats for many ground-nesting bird populations. Lodgepole pines may be good news for Goldcrest and Crossbills but not for species such as Golden Plover, Dunlin & Redshank. For breeding waders, the most obvious impact of a new forest is direct loss of breeding habitat but trees can have wider effects, by providing cover for predators and breaking up swathes of open land that are used at different stages of the breeding season. Little is currently known about how predators in Iceland use forest plantations but any perceived risks of predator presence and reduced visibility is likely to influence densities of birds in the surrounding area.

Iceland’s open habitats have suited breeding waders for hundreds of years

Aldís E. Pálsdóttir’s studied changing bird populations in lowland Iceland during her PhD at the University of Iceland, in collaboration with researchers from the University of East Anglia (UK) and the University of Aveiro (Portugal). Among the most concerning of these changes is the rapid expansion of forestry in these open landscapes.

Assessing the potential impacts of trees

In a 2022 paper in the Journal of Applied Ecology, Aldís assesses whether densities of ground-nesting birds are lower in the landscape surrounding plantations and whether these effects vary among plantations with differing characteristics. She and her fellow authors then quantified the potential impact of differing future afforestation scenarios on waders nesting in lowland Iceland.

Forestry currently covers about 2% of Iceland’s land area so the potential for growth is massive. In 2018, the Icelandic government provided additional funding to the Icelandic forest service to increase the number of trees planted, with a goal of enhancing carbon sequestration. As forestry primarily operates through government grants to private landowners, who plant trees within their own land holdings, plantations typically occur as numerous relatively small patches in otherwise open landscapes. These features make Iceland an ideal location in which to quantify the way that plantations affect densities of birds in the surrounding habitats, and to identify afforestation strategies that might reduce impacts on globally important wader populations.

To measure the effects of plantation forests on the abundance and distribution of ground-nesting birds, in particular waders, 161 transect surveys were conducted between May and June 2017. To avoid systematic bias arising from possible “push effects” of corralling birds in front of the surveyor, surveys were conducted along transects that started either at the edge of the plantation, with the observer moving away (79 transects), or started away from the plantation, with the observer walking towards it (82 transects). Please see the paper for the full methodology. The variation in density with distance from plantation was used to estimate the likely changes in bird numbers, resulting from future afforestation plans, and to explore the potential effects of different planting scenarios.

Bird communities change around plantations

Snipe densities are highest close to young forests

On the transects, 3713 individual birds of 30 species were recorded. The nine most common species (excluding gulls, which rarely breed in the focal habitats) were seven waders (Oystercatcher, Golden Plover, Dunlin, Common Snipe, Whimbrel, Black-tailed Godwit & Redshank) and two passerines (Meadow Pipit & Redwing). These species accounted for 88% of all birds recorded.

  • Of the seven waders, Snipe was the only one found in significantly higher numbers closer to plantations. Snipe density declined by approximately 50% between the first (0-50 m) and second (50-100 m) distance intervals, suggesting a highly localised positive effect of plantations on Snipe densities.
  • Densities of Golden Plover, Whimbrel, Oystercatcher, Dunlin and Black-tailed Godwit all increased significantly with increasing distance from plantations. Dunlin and Oystercatcher showed the largest effect (~15% increase per 50 m), followed by Whimbrel (~12%), Black-tailed Godwit (~7%) and Golden plover (~4%).
  • Although Redshank did not show a linear relationship with distance from plantation edges, densities were lowest close to the plantation edge.
  • There were more Redwings close to woodland edges but Meadow Pipit showed no change in density with distance from plantations.

Golden Plover, Whimbrel and Snipe were found in lower densities close to the tallest plantations (over 10 m), when compared to younger plantations (tree height 2m to 5m), suggesting that the impact of forests gets more pronounced as the trees grow. Plantation density and diameter had no additional effect on the species that were in lower densities closer to the plantations, implying that the mere presence of plantations induces the observed changes in abundance. See the paper for more details.

The bigger picture

Aldís Pálsdóttir and Harry Ewing walked every step of every transect and made detailed counts of what they saw – data that are invaluable when considering local impacts of plantations – but the paper becomes even more interesting when the authors look at the bigger picture. When plantations are distributed across these open landscapes, in different configurations, what will be the accumulated effects on the numbers of breeding waders? They estimate likely changes in abundance resulting from planting 1000 ha of plantation in different planting scenarios, ranging from a single block to lots of small patches.

  • Planting 50 smaller patches of 20 ha, instead of 1000 ha of forest in one large patch, is estimated to double the resulting decline in abundance (because there is more forest edge and hence a bigger effect on more open habitat)
  • This effect increases even further as the patches become smaller; in their models, planting 1000 blocks each of 1 ha would have nine times the impact of planting one forest of 1000 ha.
  • Proximity of woodland seems to be the driver of local distributions of breeding waders so the authors suggest that the amount of edge (relative to area) should be minimised, to reduce the impact of a plantation – which means making forests as near circular as possible.

It is clear that fewer larger forestry plots are likely to be less bad than lots of small, local plantations, in terms of the effects on wader populations. The figure below illustrates how much more land is affected when one woodland is replaced by four with the same total area. The grey area (equivalent to a 200 metre annulus) accounts for 88 hectares in the one-patch illustration and 113 hectares for four patches.

An urgent need for action (and inaction!)

Iceland holds large proportions of the global nesting populations of Golden Plover (52%), Whimbrel (40%), Redshank (19%), Dunlin (16%) and Black-tailed godwit (10%) (see Gunnarsson et al 2006) and is home to half or more of Europe’s Dunlin, Golden Plover and Whimbrel. Data in the table alongside have been extracted from Annex 4 of the report, which was discussed at the 12th Standing Committee of AEWA (Agreement on the Conservation of African-Eurasian Migratory Waterbirds) in Jan/Feb 2017.

Aldís measured the areas of 76 plantations in her study, using aerial photographs. The total area of woodland was about 2,800 ha and the total amount of semi-natural habitat in the surrounding 200 m was about 3,600 ha. Using the reduced densities that she found on the transects and the direct losses for the plantations themselves, she estimates potential losses of about 3000 breeding waders, just around these 76 forest plots. Extrapolating this figure to the whole of the Southern Lowlands of Iceland, the total losses resulting from all current plantations are likely to already be in the tens of thousands. Worryingly, the densities measured on the transects in this paper (even 700 m from forest edge) were well below those measured (slightly differently) in previous studies of completely open habitat, suggesting that losses may already be significantly higher than estimated in the paper.

A scary statistic in the paper is that “6.3% of the Icelandic lowlands is currently less than 200 m from forest plantations”. Given the incentives to plant lots more trees, this is particularly worrying for species such as Black-tailed Godwits, the vast majority of which breed in these lowland areas (between sea level and 300 metres).

Non-native trees are spreading beyond the boundary of a planned forest

It has been suggested that breeding waders might move elsewhere when impacted by forestry but migratory wader species are typically highly faithful to breeding sites. If birds are not going to move to accommodate trees, then perhaps plantations should be located where bird numbers are naturally low, such as in sparsely or non-vegetated areas, at higher altitudes and on slopes? Planning decisions could usefully be informed by surveys of breeding birds, to identify high-density areas that should be avoided.

The severe impact that planting forests in open landscapes can have on populations of ground-nesting birds emphasises the need for strategic planning of tree-planting schemes. Given Iceland’s statutory commitments to species protection, as a signatory to AEWA and the Bern Convention on the Conservation of European Wildlife and Natural Habitats, and the huge contribution of Iceland to global migratory bird flyways, these are challenges that must be addressed quickly, before we see population-level impacts throughout the European and West African Flyway.

To learn more

The take-home message from this work is clear. Local planning decisions and the ways in which forestry grants are allocated are producing a patchy distribution of plantations across the lowlands of Iceland, and this is bad news for breeding waders.

The paper at the heart of this blog is:

Subarctic afforestation: effects of forest plantations on ground-nesting birds in lowland Iceland. Aldís E. Pálsdóttir ,Jennifer A. Gill, José A. Alves, Snæbjörn Pálsson, Verónica Méndez, Harry Ewing & Tómas G. Gunnarsson. Journal of Applied Ecology.

Other WaderTales blogs that may be of interest:

Forest edges

Work by Aldís Pálsdóttir (pictured right)

Changing agricultural systems in Iceland (work by Lilja Jóhannesdóttir)

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.

Power-lines and breeding waders

Around the globe, people are consuming more energy, much of which is delivered to cities, towns, individual homes and businesses via over-head electricity cables. In a paper in Ibis, Aldís E Pálsdóttir and colleagues investigate the effects of power-lines on Iceland’s breeding waders. This is the first of several papers from Aldis’ PhD thesis, in which she seeks to understand how forestry and the sprawl of new infrastructures (roads, cottages and power-lines) are changing bird distributions within what were previously open landscapes.

Breeding waders in Iceland

Iceland is a hot-spot for breeding waders, holding half or more of Europe’s Dunlin, Golden Plover and Whimbrel, in a country that is a bit smaller then England. In a 2017 report prepared by AEWA (Agreement on the Conservation of African-Eurasian Migratory Waterbirds), in response to concerns about the effects of afforestation on Iceland’s waterbirds, we learn that:

“Iceland is second only to Russia in its importance as a breeding ground for migratory waterbirds in the AEWA region. It supports the most important breeding populations in Europe for six species of waders and is the second most important country for three.”

Data in the table alongside have been extracted from Annex 4 of the report, which was discussed at the 12th Standing Committee of AEWA in Jan/Feb 2017.

Power lines

For waders nesting in Iceland, power-lines are a new addition to a once-open landscape. Pylons provide potential nesting opportunities for Ravens and perches for Gyrfalcons, while the wires between them are a collision risk. Under power-lines, carcases of swans, geese and waders may attract scavenging Arctic Foxes and Ravens, thereby increasing the activities of nest predators. Perhaps these actual and perceived threats affect densities of breeding waders in the vicinity? Or might birds react to something less obvious, such as the emission of UV light or electromagnetic radiation?

In Iceland, the vast majority of electricity is produced from hydropower or geothermal sources, often long distances from the areas in which the power is used. A new move to develop the wind energy sector has the potential to further add to the number of power lines and introduce them in more areas of the country. Much of the increase in electricity production over the last fifty years has been used to fuel industries such as aluminium smelting and there is the potential to further expand generation capacity, perhaps exporting some electricity to other countries.

Much of Iceland’s electricity is used to power aluminium smelters (here, in Hvalfjörður)

Counting the birds

Aldís counting waders on a transect

Aldís conducted the fieldwork for this study between the 6th May and the 20th June 2019, counting birds along 85 transects of between 300 m and 500 m, running perpendicular to power lines. The full methods are described in the paper but it is interesting to see that they included a check to see whether there were different results if walking towards or away from the power lines. Each transect was divided into intervals of 50 m length, each corresponding to 1 hectare of surveyed land. For each power line, Aldís recorded the number of cables, pylon characteristics, and the height and voltage of the line.


In total, 1067 birds of 21 different species were recorded on the 85 transect surveys. Over 90% of sightings were of eight species considered in the subsequent analysis: Dunlin, Black-tailed Godwit, Golden Plover, Meadow Pipit, Redshank, Redwing, Snipe and Whimbrel. Having analysed the data, Aldís and her colleagues concluded that:

  • For all eight species combined, the areas closest to the power lines (0-50 m) supported densities of approximately 112 birds/km2 (±13 SE) which increased by approximately 58% to 177 birds/km2 (±24 SE), in the sector that was between 450-500 m away from the power-lines. On average, there was a 4% increase in abundance between adjacent 50 metre bands.
  • At the species level, Redshank (figure below) and Whimbrel density increased significantly with distance from power lines (18% and 9% per 50 m, respectively) but no other significant effects were detected for other species individually.
  • There were no detectable difference between types of power-lines or relating to the voltages of the electricity they carried.

Implications of the research

In the paper’s discussion there are questions as to why densities of Redshank and Whimbrel (right), in particular, are lower near power-lines. The two species behave differently while nesting, with Redshanks being nest-hiders and Whimbrel nesting in the open, but previous research has shown that their nest predation rates are quite similar (see Where to nest?).

The reason why significant reductions in density close to power lines were apparent for Whimbrels and Redshanks (but not for other species) is not clear but the authors suggest that sample sizes may have been too low for there to have been measurable effects for species such as Dunlin and Golden Plover (below).

Power lines could have direct impacts, such as increased collision risk, but this may be difficult to establish directly, as the authors suggest that carcasses are likely to be quickly removed by scavengers.

Ravens may find it easier to find and predate nests if there are pylons or wires on which to perch but it will be hard to discriminate between an actual predation effect, reducing numbers in areas close to power-lines, and the avoidance of risky areas because of a perceived threat of predation. This is discussed in Mastering Lapwing conservation.

Given the depressed density of ground-nesting bird species in the vicinity of overhead power lines, the authors of the paper suggest that burying power lines might be a better option, even though there would be temporary disturbance to the ground during installation.

What are the implications for Iceland’s breeding waders?

It would be interesting to calculate how many Whimbrel and Redshank (left) territories would be lost over the course of a 50 km run of power-lines through open landscapes – and then extrapolate that to 500 km and 5,000 km. As shown in the earlier table, 75% of Europe’s Whimbrel breed in Iceland. How vulnerable are they to power-line infrastructures and what might be the impact on a breeding population of over 300,000 pairs?

This is the first of several papers from Aldis’ thesis, in which she seeks to understand the current rapid changes to Iceland’s lowland landscapes. Links to other blogs and papers will be added as they appear. It should soon be possible to reveal the combined effects of these incursions into open wader habitats, by considering plans that might affect these areas over the next twenty years, working out potential losses and setting these numbers in a flyway context.

This paper is published as:

Effects of overhead power-lines on the density of ground-nesting birds in open sub-arctic habitats. ALDÍS ERNA PÁLSDÓTTIR, JENNIFER A. GILL, SNÆBJÖRN PÁLSSON, JOSÉ A. ALVES, VERÓNICA MÉNDEZ, BÖÐVAR ÞÓRISSON & TÓMAS G. GUNNARSSON. Ibis. https://doi.org/10.1111/ibi.13089

Here’s a link to another blog about Aldís Pálsdóttir’s research: Iceland’s waders need a strategic forestry plan.

A complementary set of papers by Lilja Jóhannesdóttir investigated how changes to Iceland’s farming may also be affecting breeding waders. These are discussed in three WaderTales blogs:

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.

Keep away from the trees

There are concerns that waders nesting in open landscapes are threatened by habitat fragmentation, and may be increasingly so in the future by a drive to plant more trees. Plantations obviously remove nesting space used by species such as Lapwing and Ringed Plover but they could also create hiding places for predators that can then target nests in the vicinity. In a 2022 paper in Animal Conservation, Triin Kaasiku and colleagues report on the outcomes of 753 Estonian wader nests in open areas close to forest edges along Estonia’s coastal fringe. Which are the key predators and where do they hunt?

Wide horizons

Open landscapes have been lost at an unprecedented rate over the past century. Warmer temperatures and reduced management of semi-natural landscapes provide ideal circumstances for the establishment of shrub growth and the expansion of forests. At the same time, afforestation campaigns are encouraging tree planting, to generate revenue and perhaps contribute to climate change mitigations. The combined effects of these drivers are exacerbated by the way that these processes increase the fragmentation of these already-threatened open habitats, which have traditionally provided homes for waders such as Dunlin and Curlew.

The coastal strip that used to provide important breeding habitat has nowadays often become fragmented by forest

Waders in Estonia

The semi-natural wet pastures of the Baltic coast have persisted for centuries, formed by the combined effects of floods, winter ice and postglacial land uplift, and through grazing by domestic livestock and wild geese. Together, these processes have created and preserved important wader breeding habitat. Sixty years ago, Estonia’s coastal grasslands used to be full of Lapwing, Dunlin and Redshank, as well as Black-tailed Godwit, Curlew, Common Snipe and Ruff. Since then, farmland abandonment and afforestation, both as a result of commercial forestry and natural succession, have reduced the area of coastal grasslands by about 70%.

The Boreal Baltic coastal grassland habitat is listed as a priority habitat type in Annex I of the EU Habitats Directive. Despite efforts to restore this habitat, by reducing reed and tree cover over the last two decades, most wader populations in these areas have not recovered. This is discussed in a blog about the effectiveness of agri-environment schemes in the same study area (Remote monitoring of wader habitats). Direct habitat loss is part of the problem for Estonia’s breeding waders but increased predation rates may also be a factor. Triin Kaasiku and colleagues have studied this diverse breeding wader community, to learn more about how nest predation varies across this wet grassland-forest system.

Follow that nest

Wader breeding densities at the study sites varied from 7 to 160 wader pairs per km2; the upper end is considered to be a high density, at the European level. Nests were found and then revisited approximately weekly. The Animal Conservation paper describes fully how evidence was used to determine whether nests were successful and to consider the probable causes of failures. Wader nest survival was measured during three breeding seasons (2018-2020).

Lapwing, Ringed Plover, Redshank and Dunlin were the four main species studied by Triin Kaasiku and her colleagues (see table). They accounted for 655 out of the 753 nests, with six other species providing smaller samples. To understand more about predation events, the team deployed camera traps alongside 85 of the nests, all of which were within 1 km of trees and forests.

Coastal flooding can potentially be a serious issue

About 80% of nesting attempts were unsuccessful (526 out of 655) and the outcomes of 14 other nests could not be established. Of the known failures, 89% were lost to predation, while other causes of nest failures included abandonment (6.7%), flooding (2.3%), trampling (0.2%), and others where the causes were unclear (2.3%). The seemingly high nest abandonment rate may also include some nests that were subject to temporary flooding. The results are based on 679 nests that either hatched or where there was evidence of predation.

There were no discernible differences in survival rates for different species. This is in line with findings in an Icelandic study that compared outcomes of open-nesting species (Golden Plover, Oystercatcher and Whimbrel) with those of species that hide their nests (Redshank, Black-tailed Godwit and Snipe). See Where to nest?

Nest losses

The mean daily survival rate (DSR) for nests was 0.929 which, over a combined incubation and laying period of 27 days, predicts that only about 14% of nests survive through to hatching.

DSR is higher further from the forest and where the amount of cover is lower. These two metrics are obviously related but the effects are teased apart in the paper.

From the modelled data, a nest that is only 20 m from forest edge has a 7% chance of being successful (95% CI = 5-11%), while the equivalent figure for one that is 1  km away is about 26% (18-34%).

Similarly, 3% (1-8%) of nests hatch when local forest cover (within 1 km of the nest) is about 50%, compared to 19% (15-23%) in completely open areas.

Nest success was too low across the whole study area – not enough chicks are produced

Predators and predation

Nest cameras were deployed to track what happened to 85 nesting attempts. Although 64 nests were predated, the camera trap only managed to record the nest predator in 41 cases. Of these events, 31 nests were lost to Red Fox, 5 to Golden Jackal, 3 to Raven and 2 to Badger.

Fox predation occurred on average at 217 m (95% CI=161-273 m) from the forest edge but the other mammals tended to predate nests that were further from cover. Predation events by Raven may also be more frequent closer to forest edge, but these events were rare. Based on the recordings of the camera traps, Red Fox detection rate was higher closer to the forest edge but no similar relationship was found with the proportion of forest cover.

In this Estonian study, wader nest survival did not vary with distance from smaller patches of trees or bushes (<30 m wide). Perhaps these patches may be too small for Red Foxes to hide or forage in.

The bigger picture

There is strong evidence, from studies in the UK and elsewhere, that species that breed in open habitats avoid woodland (see the WaderTales blog Mastering Lapwing conservation) and may experience greater population declines in more fragmented landscapes (as discussed in Curlews can’t wait for a treatment plan).

In the Estonian wet grassland-forestry patchworks studied by Triin Kaasiku and colleagues, Red Fox was the most commonly encountered predator operating close to forest edge. It was in these areas that eggs were most likely to be taken. This is in line with some other studies – but by no means all. The authors discuss in detail why different guilds of predators may have different effects in different circumstances and how patterns might be distorted if waders actively avoid nesting near forests or if there are complex predator-prey networks. See Further Reading below.

Predated Lapwing eggs

Hatching success is not high in any of the areas studied in Estonia – even at the lowest forest cover, only 19% of the nests hatch. This result shows that habitat fragmentation may have more severe effects on the open landscape species than previously realised. It may also indicate that more attention should be directed at the high number of generalist predators.

Conservation implications

In Estonia, as elsewhere, landowners are being encouraged to use land to grow food, to deliver biodiversity gain and to lock up carbon. Others can argue whether planting trees is necessarily a good thing for carbon capture, especially if deep peat is drained and cultivated in the process, but forestry is becoming more fashionable. This paper reminds us that piece-meal planning decisions that, for instance, provide grants to one landowner to preserve habitat for declining species of wader and grants for a neighbour to plant trees, are unlikely to maximise biodiversity benefits.

The full paper is available here:

Predation-mediated edge effects reduce survival of wader nests at a wet grassland-forest edge. Triin Kaasiku, Riinu Rannap and Peep Männil. Animal Conservation. doi.org/10.1111/acv.12774

Further reading

Foxes play leading roles in several WaderTales blogs but this selection may be of particular interest:

Tool-kit for wader conservation looks at different ways of reducing predation, particularly by foxes, within lowland wet grassland. The focus is upon issues in the UK.

Can habitat management rescue Lapwing populations? assesses whether the available tools have the power to deliver sustainable wader populations.

Trees predators and breeding waders is a cautionary tale. Reestablishing nesting habitat for species such as Dunlin and Curlew is not just a matter of removing the trees. It may take up to ten years for predator numbers to drop to levels that are associated with an open landscape.

Dunlin: tales from the Baltic is not focused on predation but fragmentation and predation are parts of the story. Veli-Matti Pakanen’s Finnish research is complementary to the studies in Estonia.

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.

Chick squeaks

During the first few weeks of life, wader chicks rely upon their parents to take them to good feeding spots and to look out for potential predators. These youngsters will need to come when called and to freeze or hide when a crow flies over or a fox is on the prowl. A wader chick communicates with its parents too, so that they know where it is and can be alert to distress, but are there more subtle messages being communicated and why do male and female chicks produce discernibly different calls?

Kristal Kostoglou has been listening to the chicks of Red-capped Plovers and Southern Masked Lapwings in Australia and analysing recordings of their calls. Her research is presented in a 2022 paper in Ibis.

Handling waders

Broken-wing distraction display by parent Red-capped Plover

Anyone who studies breeding waders will be aware that parents get very anxious if a chick that is being ringed and measured starts making alarm calls – but how many people have recorded those calls? As Kristal Kostoglou says in the abstract of the paper she has written with her coauthors “We opportunistically recorded calls of chicks while they were in the hand and analysed the calls to determine whether call structure is related to sex or body mass (a proxy for age)”. The results are fascinating.

At the start of the paper, there’s a useful summary of previous studies of the calls made by chicks. It is believed that developmental changes in chick vocalisations can allow parents to assess chick age and/or condition, with evidence that calls get deeper (frequency drops) as chicks get larger. There have been a couple of previous wader studies, relating to Pied Avocet and Eurasian Stone-curlew. In the Ibis paper, Kristal Kostoglou investigated potential relationships of call traits to body mass and sex in chicks of two shorebird species; Red-capped Plover and Southern Masked Lapwing.

The two study populations

The calls of Red-capped Plover chicks were recorded in the Cheetham Wetlands (Victoria, Australia) and the Southern Masked Lapwings were studied on Phillip Island (Victoria, Australia). Parents brood and defend their chicks which fledge at about 35 days (plovers) and 45 days (lapwings) respectively. Nests were found during incubation and most chicks were ringed just after hatching. Each chick was measured & weighed, a recording was made, and a small amount of blood was taken, to establish sex.

Recording the sounds made by a Red-capped Plover chick, as it is weighed and measured

Sound recordings were made using a portable digital recorder and an omnidirectional microphone. The 26 plover chicks made between 1 and 248 calls each and the 95 lapwing chicks produced between 1 and 336 calls. Data were collected from 9 female plovers and 17 males, between from the day of hatching and approximately 4 weeks of age. For lapwings the equivalent figures were 46 females, 49 males and up to 5 weeks of age.

Six call traits were analysed: call duration, the time between calls, entropy, minimum dominant frequency (kHz); dominant frequency range (the difference between a call’s minimum and maximum dominant frequency); and frequency modulation. Please see the Ibis paper for more details and technical information about the analyses. This related paper may also be of interest: Anatomy of avian distress calls: structure, variation, and complexity in two species of shorebird.


For Red-capped Plovers:

  • Males were more vocal – time between calls was shorter for males than for females.
  • Heavier (and hence older) chicks called more frequently.
  • As mass increased, the dominant frequency range of calls decreased (calls became less shrill).

For Southern Masked Lapwings:

  • For both sexes, dominant frequency range decreased with increasing body mass (calls became less shrill with age).
  • The decline in dominant frequency range was greater in males, resulting in a lower dominant frequency range than for females. This meant that the difference between the calls of the two sexes became more discernible over time.
  • Frequency modulation was lower for males than for females.
  • As body mass increased, frequency modulation and entropy of lapwing calls decreased.

In the Discussion, the authors consider how a heavier bill and changes to the structure of the upper vocal tract might be linked to the results. It has been suggested that chicks modify sound output to utter more adult-like calls, as they get older.

Red-capped Plover chick

A faster repetition rate of distress calling, as observed in Red-capped Plovers, might encourage parents to provide more defensive support for male chicks, which could contribute to the higher survival of male over female chicks, as reported for several plover species.

For Southern Masked Lapwings, there appeared to be sex-linked differences in calls from hatching, with these differences getting more marked with age. This could mean that either the voice anatomy develops differently or that vocal control is different in male and female chicks.

Conservation implications

Southern Masked Lapwing chicks, at the point of hatching

Both Red-capped Plovers and Southern Masked Lapwings are considered to be of ‘least concern’, according to the IUCN/BirdLife conservation criteria. Many other waders, around the globe, are under various levels of threat, with predator pressure being a significant cause of decline for some species. The ability of adults and chicks to stay together through to the fledging of the chicks could be particularly important, in these cases, with communication being key to success.

Kristal Kostoglou’s study analysed distress calls, which she and others suggest may be under the influence of natural selection. She points out that some non-distress vocalisations, such as contact calls, might communicate further information about the caller’s sex. It is also possible that distress calls may serve to communicate with siblings or with other chicks, not just with parents. In the paper, the authors suggest that future studies could investigate associations between shorebird chick calls and sex, using the full repertoire of chick calls and across species whose adult call repertoires and characteristics vary between sexes. There were few recaptures of chicks in this study and the authors suggests that it would be interesting to observe how calls of individually-marked chicks change over time.

A passing thought

Head-started Black-tailed Godwit chicks have no contact with their parents

One issue that might be considered by others is how chick calls develop if there are no parents in attendance, as we see if chicks are head-started. There is a growing movement to support populations of threatened wader species by removing first clutches of eggs, incubating them and then rearing the chicks in captivity. In their pens, they have no contact with adults and communicate only with each other.

It is clear that head-starting has worked for Spoon-billed Sandpipers, a species that was heading for extinction as discussed here, and the early signs are good for England’s breeding Black-tailed Godwits but do these hand-reared individuals miss out in some ways? Potentially the ‘language’ developed within the family may be important when the chicks are themselves parents? That’s not going to be easy to test! However, perhaps it might be possible to see if there are any differences in the development of calls between hand-reared and parent-reared chicks of the same species?

Communication between parents and chicks helps to keep the family together

In summary

The study published in Ibis provides the first evidence for charadriid chicks of (a) a sex difference in call structure and rate and (b) gradual growth-related changes in call structure and rate, across chicks. The detailed write-up will hopefully be useful in further studies of shorebird vocalisations during growth, which may help further to explain the development and functional significance of all that squeaking!

Vocal traits of shorebird chicks are related to body mass and sex. Kristal N. Kostoglou, Edward H. Miller, Michael A. Weston and David R. Wilson. Ibis. https://doi.org/10.1111/ibi.13055

Red-capped Plover chick seeks to hide in the cracked mud, using the shadow to break up its outline

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.

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.


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.

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.


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.