Why count shorebirds? A tale from Portugal

The Sado Estuary is one of Portugal’s most important wetlands – a key link in the chain of sites connecting Africa and the Arctic, on the East Atlantic Flyway. In a paper in Waterbirds, João Belo and colleagues analyse changes in numbers of waders wintering in this estuary over the period 2010 to 2019, with a focus on roost sites. These results are interpreted in regional and flyway contexts. The team find serious declines in numbers of Avocet, Dunlin and Ringed Plover.

Lost roost sites

The Sado Estuary became a nature reserve in 1980 and has since been classified as a Ramsar Site, a Special Protection Area (SPA – Natura 2000) and an Important Bird and Biodiversity Area. The Estuary lies about 30 km south of the much larger and more famous Tagus (or Tejo) Estuary, which is threatened by a new international airport (see Tagus Estuary: for birds or planes?). A suggestion that the Sado could provide mitigation habitat for damage done to the Tagus prompted scientists from the Universities of Aveiro and Lisbon to review the current state of the estuary.

João Belo and his colleagues used monthly data from a programme of wader surveys, conducted largely by volunteer birdwatchers. These took place between January 2010 and December 2019. Roosting birds were counted at high tide along the northern shores of the Sado estuary and any habitat changes were noted.

During the ten-year survey period, 21% of the available high-tide roost area was lost. These changes were associated with the commercial  abandonment of saltpans (with consequent increases in vegetation) and the conversion of others for fish farming (often with netting, to keep out fish-eating birds).

Results of the survey

In their paper, João Belo and colleagues focused on total numbers of waders and counts of the six most commonly-encountered species: Avocet, Dunlin, Black-tailed Godwit, Redshank, Ringed Plover and Grey Plover. They compared winter (Dec, Jan and Feb) counts in 2019 with those from 2010. This is when peak numbers of Avocet, Redshank and Grey Plover occur. Higher counts of Dunlin are made in spring, as schinzii birds returned from Africa, with peak counts of Black-tailed Godwit and Ringed Plover occurring in autumn.

The key findings are:

Ringed Plover numbers dropped by 23%

• There was a strong decrease in the overall number of waders wintering in the Sado Estuary. This trend is mostly driven by steep declines in three of the six most abundant species: Avocet, Dunlin and Ringed Plover.
• Avocet numbers were 42% lower in 2019 than they had been in 2010.
• Dunlin numbers dropped by over half, with 2019 counts being only 47% of those in 2010. These are mostly dunlin of the alpina subspeciesthat breed between Northern Scandinavia and Siberia.
• Ringed Plover numbers dropped by 23% between 2010 and 2019.
• Redshank increased significantly between 2010 and 2019, while the population of Grey Plover was relatively stable, and it was not possible to derive a population trend for Black-tailed Godwit.

It is interesting to look at these patterns alongside data collected in Britain & Ireland, over the same period. As discussed in Do population estimates matter? and Ireland’s wintering waders, there have been major changes in wader numbers, with most species currently in decline.

At the same time that Avocet numbers have dropped on the Sado Estuary they have rocketed in the UK (here seen on the Humber Estuary)

• Winter Avocet numbers have increased massively in Britain & Ireland. It is possible that young birds are more easily able to settle in these northern areas, now that winter temperatures are generally warmer. Declines in Portugal may reflect a northwards shift of the winter population, driven by new generations of birds.
• Numbers of Ringed Plovers in Britain & Ireland did not change over the period 2010 to 2019 but had dropped a lot in the preceding twenty years.
• For Dunlin, the size of the declines in Britain & Ireland are consistent with those on the Sado. It has been suggested that more young birds might be settling in areas such as the Wadden Sea, closer to Siberian breeding areas, something that may have become more possible given the reduced intensity and occurrence of freezing conditions along the east coast of the North Sea.

Regional and Flyway patterns

As discussed in the blog Interpreting changing wader counts, based on research led by Verónica Méndez, local changes in numbers are usually reflective of broader changes in population levels. Individual waders are unlikely to seek alternative wintering sites unless habitat is removed, so birds do not re-assort themselves into the ‘best’ areas when population levels decrease. Instead, there is general thinning out across all sites as populations decline. In this context, it is unsurprising that the trends in the Sado Estuary are similar to those found elsewhere in Portugal and in other Western European wintering areas.

Looking forwards

The survey data collected between 2010 and 2019 form a useful backdrop against which to monitor what might happen when (or perhaps if) a new international airport is constructed within the nearby Tagus Estuary. If some birds are displaced to the Sado, increases in numbers might be expected.

Displacement is not cost-free, as has been shown in a well-studied population of Redshanks on the Severn Estuary in Wales. When Cardiff Bay was permanently flooded, as part of a major redevelopment, colour-marked Redshank dispersed to sites up to 19 km away. Adult birds that moved to new sites had difficulty maintaining body condition in the first winter following the closure of Cardiff Bay, unlike the Redshank that were already living in these sites. Their survival rates in subsequent winters continued to be lower than for ‘local’ birds, indicating longer-term effects than might have been predicted.

These three papers are essential reading for anyone interested in the consequences of displacements caused by development projects.

• Burton, N.H.K. 2000. Winter site-fidelity and survival of Redshank at Cardiff, South Wales. Bird Study 47: 102–112.
• Burton, N.H.K. & Armitage, M.J.S. 2005. Differences in the diurnal and nocturnal use of intertidal feeding grounds by Redshank. Bird Study 52: 120-128.
• Burton, N.H.K., Rehfisch, M.M., Clark, N.A. & Dodd, S.G. 2006. Impacts of sudden winter habitat loss on the body condition and survival of Redshank. Journal of Applied Ecology 43: 464-473

Given that the Sado has multiple conservation designations, including as a Ramsar site, and that this study has shown a clear loss of available roosting areas, perhaps it is time to identify a high-tide refuge that can be fully protected and managed in ways which create a range of suitable habitats for use by long-legged and short-legged waders. A nature reserve such as this has a potential to attract birdwatchers too, with prospective increased income from tourism.

Sado International

Curlews don’t get a mention in this paper but the The Sado provides a neat link to a 2022 blog, A Norfolk Curlew’s Summer. This tale focuses on ‘Bowie’, a male Curlew that breeds in Breckland (Eastern England) and has been tracked to The Sado Estuary. In the blog, Bowie’s story stops in The Tagus but he subsequently headed further south to The Sado, where he spent the winter. At the time of writing (13 Feb 2023) he is still there but hopefully he will heading north soon.

The Sado is not only important in the winter, of course. As mentioned earlier, it is a spring stop-over for birds such as schinzii Dunlin, heading north from Africa to Siberia, and a moulting/staging site for waders heading south in late summer. Tracking and colour-ringing are telling more of these stories, with links to countries as far north as Canada and Siberia and as far south as South Africa.

The overgrown embankments within the former saltpans no longer provide suitable roosting sites for waders

Keep counting

The Sado story could not have been written without the work of volunteer counters who collect monthly data during the winter months, on the Sado Estuary, across Portugal and on the wider East Atlantic Flyway. These monitoring efforts are essential when attempting to track changes in wader populations, especially when extra information can indicate links to habitat changes, as is the case in the Sado. The international picture is painted using Flyway information generated using January counts that are developed by the Institute for Nature Conservation and Forests (ICNF).

Here is a link to the paper:

Synchronous declines of wintering waders and high-tide roost area in a temperate estuary: results of a 10-year monitoring programme. João R. Belo, Maria P. Dias, João Jara, Amélia Almeida, Frederico Morais, Carlos Silva, Joaquim Valadeiro & José A. Alves. Waterbirds. doi.org/10.1675/063.045.0204

Birdwatchers that volunteered to survey roost-sites gather for a team photo

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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.

What happens when the mud disappears?

The Yellow Sea provides important ‘service stations’ for shorebirds on the East Asian-Australasian Flyway, especially on their way north to Russian and Alaskan breeding areas. In a thought-provoking paper in Biological Conservation, Xiaodan Wang and colleagues consider how assemblages of waders have changed, as available habitat has been lost on this flyway. Their results suggest that conservation of these migratory shorebirds will depend upon lots of local initiatives.

The Yellow Sea

As discussed in Wader declines in the shrinking Yellow Sea, based on a paper by Colin Studds et al, the number of shorebirds on the East Asian-Australasian Flyway (EAAF) has fallen significantly. The rapid decline has been linked to a sudden drop in the survival rates of adults, especially in Bar-tailed Godwit, Red Knot and Great Knot. For these three species, the proportion of adults dying each year doubled over a period of just three years, with a strong link between species’ dependence upon the Yellow Sea and the rate of decline in numbers. This study and other papers have spawned welcome conservation initiatives and collaborative research programmes in China, South Korea and the Democratic People’s Republic of Korea.

In the most recent appraisal of numbers for EAAF shorebird species that travel as far as Australia, it is estimated that the number of Far Eastern Curlew has dropped to just 35,000 individuals and that there was a 50% drop in numbers for Curlew Sandpiper and Red Knot between assessments in 2008 and 2021. These catastrophic changes show how quickly things can go wrong, especially when adults struggle to get through the annual breed-migrate-moult-fatten-migrate cycle.

In their paper, Xiaodan Wang et al compared the results of shorebird surveys on the Yellow Sea coast of China during northwards migration, between an “early” study period (1996 to 2005) and a “late” study period (2013 to 2014), at 14 stop-over sites. These are sites where large areas of tidal habitat have been destroyed.

What happened?

In their study, the authors investigated three hypotheses:

• Are trends in population numbers at stopover sites (in China) similar to those detected in nonbreeding sites (e.g. Australia)?
• Is the diversity within shorebird communities linked to habitat condition?
• How does a reduction in the area of tidal habitat affect numbers of waders and the composition of shorebird communities?

Counting shorebirds

China’s Yellow Sea region, which extends from the Yalu Estuary (Liaoning Province) in the north, to the Yangtze Estuary (Shanghai) in the south, holds the largest areas of intertidal mudflats in the world. The first comprehensive spring shorebird surveys along this Yellow Sea coast were conducted between 1996 and 2005 (the early period in this study). The Yellow Sea coast was surveyed again in 2013 and 2014 (the late period), by which time large areas of several intertidal flats had been ‘claimed’ and turned into farmland and industrial zones. See paper for methods.

A total of 14 sites were surveyed at similar times, in the early and late periods, enabling comparisons of community richness, the abundance levels of individual species and the density of birds across the available feeding habitat. The key results of the study are:

Counts: The measured decline in bird numbers was on a scale which was less dramatic than those reported from Australia. At the 14 stopover sites, a total of 668,995 individuals of 44 species were recorded in the early period, and a total of 617,146 individuals of 43 species were recorded in the late period. This corresponds to a 7.8% reduction between the early and the late periods.

Species richness: There was much variation between sites, with between 18 and 38 shorebird species recorded, but the numbers of species on each study site were similar between the two periods.

Distributions: Decreased abundance was recorded at ten sites and increased abundance was recorded at four sites. The number of birds on the Liaohe Estuary trebled between the early and late periods, mostly due to the discovery of a flock of 80,000 Great Knot. A recent population assessment of shorebirds of the East Asian-Australasian Flyway suggests that this large flock accounts for 20% of the world’s Great Knot. These passage flocks of Great Knot tend to stay for only a few days, making it hard to reliably pick up any changes between years.

Change across the region: No significant differences were found in species richness or abundance at the same sites, between the periods, but there were significant differences among sites. Ten species were recorded in smaller numbers, with increases for Bar-tailed Godwit (up 16.8%) and Great Knot (57%).

Communities: Bird assemblages at each site were similar in the two study periods, with the commonest species remaining the same. For example, Great Knot and Bar-tailed Godwit are the dominant species in the Yalu Estuary, while Red Knot and Curlew Sandpiper are the dominant species in North Bohai Bay, in both periods.

Loss of habitat: The total tidal flat area decreased by 35.6%, from 4017 km² in the early period to 2588 km² in the late period. Habitat loss was detected at all of the 14 sites. More than half of the tidal flat was destroyed at Northern Bohai Bay, Laizhou Bay, Southwest Bohai Bay, and Jinzhou. Relatively small losses (< 10%) occurred at Chongming Dongtan, Yalu Estuary, Jiaozhou Bay, and Liaohe Estuary (see map).

Shorebird densities: Shorebird densities (birds per km² of mudflat) differed markedly between sites. During the study period, the estimated bird density increased at 7 sites and decreased at 5 sites, with an overall increase in density of over 40% between the early (1996-2005) and late (2013-2014) periods.

Bit of a squeeze?

‘Land claim’ – turning mud flats into farmland and industrial areas – reduced the amount of available feeding habitat in the Yellow Sea study area by 35% between 1996-2005 and 2013-2014. If the same numbers of waders were to be accommodated, then that would imply increases in densities of about 55%. This assumes that feeding conditions remain the same and that spring fattening can happen just as quickly, despite the higher densities. If it’s harder to find food, birds will potentially need to stay longer, to fatten up for the next part of their migration, and that suggests that densities would need to be even higher. The observed change in density of 40% is lower than 55% but this may be consistent with falling populations.

What might be happening?

It is clear that wader numbers in the Yellow Sea have declined, as expected from annual counts in Australia and New Zealand. However, the decline in spring numbers is not as dramatic as might have been predicted. The remaining birds have much less space in which to feed and densities have increased, suggesting that there will be more competition for resources than there was a decade previously. Returning to the three hypotheses put forward by the authors:

Are trends in population numbers at stopover sites (in China) similar to those detected in nonbreeding sites?

Bird surveys covered the same zones and were conducted on similar dates in the early and late periods. Changes in shorebird counts during the study periods did not always exhibit the same population trends as those at nonbreeding sites in Australia and New Zealand.

Is the diversity within shorebird communities linked to habitat condition?

The fact that there are different mixes of waders on different estuaries is probably linked to different food resources. It would be good to know more about the food that is available to species as diverse as small sandpipers and large curlew – especially in the earlier period before so much habitat was lost. Red Knot and Great Knot give hints that waders are found on sites that are appropriate to their needs, with more Great Knot on Yalu Estuary, where the key shellfish they eat (Potamocorbula laevis) are larger, and more Red Knot at Nanpu, where the shellfish are smaller. Given the expected links to available feeding opportunities on individual sites, it is unsurprising that the diversity of shorebird communities has tended to remain the same.

How strong is the link between the change in the area of tidal habitat lost and the changes in the numbers of waders and the composition of shorebird communities?

The research team expected to find changes in shorebird numbers and species composition but the relationships are not as strong as might have been expected, given the areas of mudflat that have been lost.

There is no clear pattern of community change, as just discussed, and this is in line with analyses of more comprehensive annual Wetland Bird Survey data, collected in the United Kingdom. As population levels fell, the changes were spread across all estuaries. Some people may have predicted that distributions would have become more focused on ‘better’ estuaries but this was not the case for wintering waders. Ringing studies suggest that individuals’ site-fidelity is strong, which reduces the potential for distributional change. This study is described in the WaderTales blog, Interpreting changing wader counts, which summarises a paper in Diversity & Distributions, by Méndez et al.

In the Studds paper, there is a clear signal that species that are more dependent on the Yellow Sea exhibited the steepest declines in numbers, so it seems unlikely that there was an excess of available food at the time of the first surveys (1996-2005). If food supplies are limiting, why is the link between habitat lost and numbers not stronger?

The latest estimate of the flyway’s Far Eastern Curlew population is just 35,000

Xiaodan Wang et al discuss why they did not see a clearer relationship between mudflat loss and changes in shorebird numbers. They suggest that population declines may be less severe in some (or even all) of the study sites than might have been expected because these are known hot-spots for waders. Losses may have been more dramatic elsewhere. We know that severe habitat loss can remove most waders from a site, leading to redistribution of affected birds.

It is also possible that individual birds could be staying longer at spring staging sites in the Yellow Sea, due to poorer feeding conditions or changes to the timing of migration. This build-up of birds could mask the scale of the real declines in numbers using Yellow Sea mudflats. In the future, information gleaned from tracking and through resightings of colour-ringed birds should help to monitor stop-over times. Sadly, similar data are not available for the period 1996 to 2005.

Conservation Implications

This paper provides another reminder that structured surveys are immensely valuable. Back in 1996, could anyone have imagined how quickly China’s Yellow Sea mudflats would be turned into farmland and industrial complexes? Extending surveys to more estuaries in the East Asian-Australasian Flyway, using standardized protocols and sharing datasets, will be important, when assessing responses to habitat changes at breeding, stopover, and nonbreeding sites.

Studies at the species level, especially those focused on habitat specialists, have found that some birds cannot successfully relocate to alternate sites if their major staging sites are lost (e.g., Moores et al. 2016). This paper, by Xiaodan Wang et al, concludes that maintaining populations of migratory species along the East Asian-Australasian flyway depends upon the conservation of an extensive network of estuaries within the Yellow Sea.

Paper

The full paper can be accessed here:

Impacts of habitat loss on migratory shorebird populations and communities at stopover sites in the Yellow Sea. Xiaodan Wang, Ying Chen, David S.Melville,  Chi-Yeung Choi, Kun Tan, Jiajia Liu, Jing Li, Shoudong Zhang, Lei Cao & Zhijun Ma. Biological Conservation.

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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 Whimbrel’s year

There’s a lot to fit into twelve months if you’re a Whimbrel. In the last paper from his PhD, Camilo Carneiro assesses whether Icelandic Whimbrel can always manage to complete the annual cycle of migrate-breed-fatten-migrate-moult-fatten in just 365 days.

YY-LL (Yellow yellow – lime lime) collected data for several years.

What happens if a pair of Whimbrel loses a first clutch and a successful, late breeding attempt delays departure from Iceland, for instance? Is there the flexibility to make up for lost time during a west-African winter? Some answers are provided in a paper by Camilo Carneiro et al in The American Naturalist.

Facing the consequences

We know that large waders sometimes take a year off from breeding, as discussed in Teenage waders, suggesting that they may not always have the resources they need to breed every year. This blog, based on a paper by the Bird Ecology Lab team in Chile, tells the tale of a few Hudsonian Godwits that headed for the pampa grasslands of Argentina in spring, instead of migrating to Alaska.

As discussed in Gap years for sandpipers, taking a year off may make no difference to expected lifetime breeding output and could be more common when individuals spend the non-breeding season a long way from nesting areas or in poor quality sites. If Iceland’s Whimbrel are able to compensate for any delays they face during the course of the annual cycle then perhaps that suggests that all is well for this population that migrates all the way to West Africa at the end of the summer breeding season?

The story so far

Whimbrel only spend three months of the year in Iceland, with the rest of the time spent some 6000 km further south. In July or August, an adult will exchange the sparsely vegetated river plains and night-time frosts of Iceland for mud, mangroves and tropical temperatures. The diet changes too, from terrestrial invertebrates and crowberry to crabs.

Camilo Carneiro started a PhD on Iceland’s breeding Whimbrel in 2015, continuing the work of his supervisors, Tómas Gunnarsson of the University of Iceland and José Alves from the University of Aveiro (Portugal). Using geolocator tags he investigated the capacity for Whimbrels to undertake non-stop journeys, demonstrating that autumn migration was generally direct but spring migration for most birds included a stop, often in Ireland or the UK. You can read more in Iceland to Africa, non-stop. Birds that need to (or choose to) take a break delay their arrival in Iceland by about ten days.

Subsequent papers by the same team have shown that the most consistent point of the annual migration story is departure from Africa and shown links between weather and phenology. These two papers have been covered in the WaderTales blogs Whimbrel: time to leave and A Rhapsody of Whimbrel. In an attempt to discover the best places to spend the winter months, Camilo analysed tag and colour-ring data to work out links between conditions experienced in wintering locations and subsequent breeding success, as discussed in the blog Winter conditions for Whimbrel.

Seven years of data

To investigate how migratory animals navigate their annual schedule, and where and when they can make adjustments to their timings, Camilo Carneiro and his colleagues used annual-cycle data of 38 Icelandic whimbrels tracked over 7 years. They asked three questions:

• Does the change in the timing of one event in the annual calendar, such as a late breeding season, affect the timing of subsequent events, perhaps with further down-stream domino effects?
• Can individuals compensate for delays, on migration for instance, by spending less time on the next stage of the annual cycle, e.g. by reducing a stop-over?
• Are there potential fitness consequences? Do birds that are subject to delays breed later? In waders, earlier chicks are more likely to recruit to the breeding population so being just a few days late returning to Iceland may have consequences.

During the period 2012 to 2018, a total of 78 geolocators were deployed on Whimbrel breeding in Southern Iceland. The device was attached to a leg-flag in one year (see picture) and usually collected in the subsequent breeding season. In most cases, a replacement geolocator was fitted, in order to collect further data on that individual. Unsurprisingly, it became harder to catch tagged birds over time, as birds learned to recognise nest traps and research vehicles. The fact that so much valuable information was collected from the same birds is testament to Camilo’s patience. Sixty-six geolocators were retrieved from 39 individuals. Birds could only be caught when incubating a full clutch of eggs so nest losses due to predation affected the likelihood of recapture.

Please see the paper in The American Naturalist for full details of the methods used to collect data on breeding success and for interpretation of data collected using geolocators.

Whimbrel YY-LL

Before looking at the results in the paper, here’s an example of data collected using geolocators, for two years in the life of YY-LL (Yellow Yellow – Lime Lime), pictured alongside.

In 2015, YY-LL nested successfully and left Iceland on 16^th August. After four days of direct flight, he reached Guinea-Conakry. His nesting attempt in 2017 was unsuccessful and he migrated south a little earlier, on 5^th August, again taking four days to fly 6000 km.

In the springs of both 2016 and 2018, YY-LL left the winter grounds on 22^nd April, arriving in Ireland on 26^th April in 2016 and on 25^th April in 2018. Migration from Ireland to Iceland took place between 5^th and 7^th May in 2016 and between 7^th May and 9^th May in 2018.

YY-LL hints at strong consistency of spring migration timing, independent of nest success.

Results

The 38 tagged Whimbrel provided information about 76 autumn migrations and 60 spring migrations. Most of the birds (89%) spent the winter between Senegal and Sierra Leone, particularly in Guinea-Bissau and Guinea, with one bird in Portugal and the rest in coastal northwest Africa.

As is usual in waders, females left Iceland before males, the difference being typically around six days. Although failed breeders did depart earlier than birds that successfully reared chicks, the difference was not great, again averaging around six days.

There was no suggestion that birds that left Iceland late in the season ended up on a delayed schedule for return to Iceland in the subsequent spring. This implies that resources in Africa were sufficient to ‘catch up’ with earlier birds, despite the need to complete a full moult and to prepare for another 6000 km migration.

As indicated in previous papers (and in the blogs mentioned above) spring departure dates of tagged birds from Africa were not different for different countries or for different sexes. However, birds that stopped in Europe tended to leave Africa earlier (19^th April on average) than those that made direct spring flights (30^th April). These latter birds tended to arrive in Iceland about a week earlier than birds on a two-stage migration, representing a neat, overtake manoeuvre! Typically, males arrived in Iceland about a week earlier than females.

For the sample of tagged birds, neither the autumn departure date from Iceland nor wintering location had any apparent effect on the arrival date in the next spring.

There seems to be a strong signal that, just as YY-LL did, an individual Whimbrel can make up for any delays incurred, with birds that arrive at a location later spending less time there, whether that be a wintering site or a spring stopover location. Females tended to spend longer at spring stopover locations, which ties in with the earlier arrival of males in Iceland. The graph alongside illustrates these two points. Birds that left Africa earliest spent more than 15 days at stopover sites but birds on a later schedule stopped off for as few as 6 days. Triangles represent males and squares are females.

Over the course of a year

Putting this all together, Camilo and his colleagues found that individuals appear to use the wintering sites to compensate for delays, these mostly having been associated with a successful previous breeding season. The wintering season is up to 38 weeks long so a Whimbrel that heads south a little late has plenty of time in which to catch up with earlier birds. The timings for other wader populations that spend shorter periods in wintering locations may be more constrained, given that post-breeding moult might take 20 weeks and the time to fatten up for migration can add an extra seven weeks.

Once a bird leaves Africa, it is harder to compensate for delays, although attempts are made to do so, with later individuals stopping for shorter times at spring stop-over sites and then nesting shortly after arrival in Iceland. Data in the paper suggest that it is not possible to catch up completely, before the start of the breeding season, if time is lost on the way north. Given the known link between lay-date and nesting success, these spring delays may have consequences for productivity and reduce the capacity to re-nest following clutch loss.

Iceland’s Whimbrel

Camilo’s research suggests that adult Whimbrel in the south of Iceland have the capacity to make up for any delays that they face during the annual cycle. The same may not be true for other large waders, the populations of which are mostly in decline. The blog Why are we losing out large waders? reminds us that two curlew species are thought to be extinct or on the verge of extinction and that most of the rest are in trouble.

There are increasing pressures on Whimbrel breeding in Southern Iceland, associated with new forestry plantations and infrastructures such as road developments and power lines

All is not necessarily well for Iceland’s waders either. Two 2022 papers by Aldís Erna Pálsdottir, looking at the effects of forestry and power-lines, suggest that the Whimbrel is one of the wader species most seriously impacted by ongoing changes to the Icelandic landscape, as discussed in Power-lines and breeding waders and Iceland’s waders need a strategic forestry plan. These pressures seem to be reflected in poorer breeding output, as suggested by counts of family parties of Whimbrel made by Tómas Gunnarsson and colleagues in annual June and July surveys.

Camilo’s paper may indicate that adult Whimbrel can cope with all that life throws at them but if they cannot raise enough chicks the species will still be in trouble. With financial support from the Icelandic Centre for Research, Camilo is now studying the challenges that chicks face, as they prepare for their first migration from Iceland to Africa.

Here’s the link to the paper:

Annual schedule adjustment by a long-distance migratory bird. Camilo Carneiro, Tómas G. Gunnarsson & José A. Alves. The American Naturalist. doi.org/10.1086/722566

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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?

Fieldwork

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

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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 Norfolk Curlew’s summer

This blog follows the fortunes of one Curlew (B/OW-OY) as he attempts to raise chicks in Norfolk, migrates to Portugal, diverts to avoid a forest fire and returns to his winter home in a Portuguese estuary. Using tagging data we know that he left his Norfolk breeding territory in England on 17 June 2022 and arrived at Montijo, in the Tagus Estuary of Portugal, on 16 July – but that’s only part of his story.

Curlew migration

B/OW-OY (named ‘Bowie’ in a BTO press release) may have been travelling between Norfolk and Portugal for a decade or more, flying backwards and forwards between the dry grassland of inland East Anglia and the coastal mud of the Tagus Estuary. We have new insights into his life because he was ringed and tagged on 2 April 2022, as part of a project to understand Curlew breeding success rates in East Anglia – about which more later!

Curlew migration is complex. The birds that we see in autumn and winter in the UK include some local birds but the majority will have crossed the North Sea, particularly from Finland. The map alongside is taken from the BTO website. As you can see, some British-bred Curlew migrate south for the winter. At the Europe-wide scale, the major axis of migration is also from northeast to southwest, as you can discover using the European African Bird Migration Atlas mapping tool. ‘Bowie’ may be feeding in Portugal alongside birds that have bred in Poland, Finland and France.

Breckland Curlews migrate in all directions in the autumn. At least three have been found wintering on the Norfolk coast, five colour-ringed birds have travelled to southwest England (Dorset, Devon and the Severn Estuary), another of the tagged birds has flown to Ireland and a colour-ringed bird has been seen in northern Spain.

Breckland Curlew

Harry Ewing is studying Curlew in Breckland – the tapestry of heath, grassland, forestry, arable farming and military bases in the dry, heart of East Anglia. His PhD at the University of East Anglia is being undertaken in partnership with BTO and with input from RSPB. Harry is working across a range of habitats that are used by nesting Curlew, from airfields to sugar beet fields. The breeding site of B/OW-OY is a grassland heath that is managed in ways that maintain habitats that are suitable for scarce plants, invertebrates and birds. Here, pairs of Curlew breed alongside Stone-curlews and Woodlarks.

Most of Harry’s Curlew struggle to fledge any chicks, as is the case in many places across Britain and Ireland, and it will be interesting to see the papers that come out of his PhD, in due course. Two previous WaderTales blogs describe the predation problems Curlews face in East Anglia (Curlews and foxes in East Anglia) and give information about conservation interventions that suit breeding waders (Grassland management for Stone-curlew).

Catching Curlew

During his PhD, Harry has found up to 70 Curlew nests in a single breeding season and followed their outcomes until the eggs or chicks were predated or through to fledging. Harry checks on his breeding Curlew on an almost daily basis but does not always see adults on territory. By catching birds at roost, at the start of spring, and deploying a small number of GPS tags, funded from the BTO Curlew Appeal and by Natural England, he hoped to learn more about how birds use surrounding habitats and to get more accurate information about the timing of breeding events. The extra data about migration that he gets from birds like B/OW-OY are a bonus.

Harry had been looking for opportunities to catch Curlew in the Brecks when they returned to the areas in the spring. A flock of roosting birds was spotted by local graziers on 31 March 2022 and, with the help of the Breckland Farmers Wildlife Network, Nigel Clark and the Wash Wader Research Group, a catch was made on the evening of 2 April. Eleven birds were caught, all of which were colour-ringed and eight of which were also fitted with GPS tags by Sam Franks of BTO (see picture).

A ‘miracle chick’

Harry spotted B/OW-OY on the day after capture, on a grass heath just 3.7 km away from the ringing site. As luck would have it, he was paired with B/OW-OL, a newly colour-ringed female. Harry could not download data from the tagged bird, due to a lack of signal coverage, but he saw OY and OL almost daily over the next two months. Information collected on OY’s tag and sent from Portugal in July filled in some of the gaps. OY fed mostly on cultivated plots (as described in Grassland management for Stone-curlew) but he also used nearby arable farmland, especially an asparagus field about 3 km from the heath, and made a brief trip to a local airfield during the pre-breeding period.

Both OY and OW were observed regularly and Harry found their nest and a completed clutch on 6 May. As can be seen in the photograph, it was in a very open site, on dry, grazed grass heathland. When Harry checked the nest on 25 May, three chicks had hatched and one was still hatching. A second check the next day, to confirm successful hatching of the final chick, revealed a sad tableau of overnight predation: one chick was dead, one was injured and two were missing. Injuries suggested a mustelid attack – probably a stoat (see photograph above). That was the end of the breeding attempt – or was it?

Another day later, on 27 May, Harry discovered a bird he named ‘miracle chick’; a one-day-old youngster that was feeding near the nest site, on its own, with no parents guarding it. OY and OL were still present on the heath but had joined a flock of failed breeders, presumably having assumed that their chicks had all perished. Two days later, ‘miracle chick’ had moved 100 metres and latched onto a family of two 25-day-old chicks, feeding with them and sharing the protection afforded by their parents. OY and OL were still in the post-breeding flock, presumably unaware that their breeding season had not been a total failure.

Curlews gather in small flocks when they lose their chicks or nests. For them the season is over.

As Harry continued his daily checks of broods around the Brecks, to try to understand more about the predation of chicks, he was able to establish that the first step-sibling of ‘miracle chick’ was predated on the night of 31 May/1 June and the second on the next night. Nocturnal predation of big chicks is probably by foxes. ‘Miracle chick’ continued to be defended by its step-parents until 16 June. On 17 June, Harry found an adult female with a broken wing and the chick was never seen again. Perhaps she was the step-mum and the injury was sustained in unsuccessful defence of ‘miracle chick’. ,We shall never know. By this time, OL and OY appeared to have left the heath; the last sighting of the female was on 10 June and the tagged male was not spotted after 16 June.

Heading south

Thanks to the information that was transmitted from Portugal, we know that B/OW-OY (Bowie) left Breckland at 21.30 (UTC) on 17 June, arriving on the Essex coast at Brightlingsea just 80 minutes (and about 50 miles) later. He spent the next four weeks feeding around the coast of Mersea Island, leaving at 20.00 on 14 July. There was a nine-hour lay-over in Spain on 15 July, before another evening departure (20.40). The last part of the journey is interesting; it included what looks like a diversion around the forest fires that were burning in the Ourém region in Portugal at that time (see map).

It is fascinating to think about how this one Curlew from Norfolk might have travelled to Portugal. Did it fly to the coast alone? Did it meet up with other birds that were heading south from Essex and fly in a flock? Did it ‘change flocks’ in Spain, as we would planes at an airport? This blog, based on a recent French paper, gives a feeling of what it is like to be a migrating Curlew: The flock now departing.

Threatened winter home

Bowie arrived on the Tagus Estuary at 09.30 on 16 July and he may well stay here until next February or even March. The Montijo peninsula, which appears to be his winter home, is one of the most important parts of the Tagus Estuary for Curlew. They feed in the mud and roost on salting islands, following the rhythm of the tides. This estuary is right next to Montijo Air Force Base.

The saltmarsh at Montijo on 18 July 2022. Josh Nightingale counted 22 Curlew.

Montijo Curlew are accustomed to the relatively infrequent take-offs and landings by military aircraft, in the same way that Curlew on RAF bases in East Anglia are happy to breed on the grassland alongside runways. All this will change, however, if the plan to expand the site, lengthen the runways and introduce all-day flights of passenger aircraft goes ahead (planned to operate at 5 minute intervals). Add on control measures to reduce the potential of bird-strikes to aircraft and Montijo and the Tagus as a whole will become much less attractive to hundreds of thousands of waterbirds. As you can read in Tagus Estuary: for birds or planes, we need to fight very hard to stop this new airport which is on a specially protected Ramsar site. If this jewel on the East Atlantic Flyway can be degraded then what is there to stop similar developments elsewhere?

One tagged bird

It looks as if B/OW-OY skirted around forest fires in the Ourém region.

In the spring, B/OW-OY will return to his Breckland breeding site, hopefully to meet up with B/OW-OL and to have another go at raising chicks. For him nothing has changed – he’s doing the same again for another year. The only difference is that he is tagged now, and that provides us with insights into his world and the challenges that he and other migratory waders face. Imagine what it will be like if, next year or in the near future, he arrives back in Montijo and finds bulldozers and freshly-poured concrete, instead of mud and saltmarsh. With a new airport to contend with and yet more anthropogenic change to his world, life does not look as if it will get easier.

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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.

Whimbrels arrive in Iceland

There is something magical about standing in Eyrarbakki in South Iceland, in spring, watching small flocks of Whimbrel come in over the sea. Thanks to geolocators and satellite tags (about which more later) we know that these amazing waders will have been in the air for around five days, since leaving the west coast of Africa.

Looking down at the seaweed-covered rocks on the morning of 27^th April 2022, we could pick out small groups of new arrivals. Most were resting but one bird was looking for crabs, just as it had been doing in West Africa just a few days previously. A few birds headed off inland while we were watching, making the distinctive seven-note whistle as they left. It sounded almost like a cry of “made it”!

Incoming Whimbrel: Tómas Gunnarsson

Setting the scene

We had been here before. On 22^nd April 2008, I did a live broadcast for the BBC Radio 4 programme World on the Move from this very spot, when I described visible migration to Brett Westwood. On that day, we could see Purple Sandpipers and Turnstone feeding on the tide edge, White Wagtails and Meadow Pipits, newly arrived from Iberia, and small flocks of Golden Plover flying in low over the sea. During the programme, a gaggle of 35 Pink-footed Geese flew in strongly from Britain, as did four Arctic Skuas that had spent winter in the Southern Hemisphere. You can listen to the programme HERE.

On 27^th April 2022, the skies were not as busy as they had been fourteen years previously. High pressure over the British Isles and northerly winds across the Atlantic seemed largely to have pressed ‘pause’ on migration from Britain & Ireland. There were small numbers of newly-arrived White Wagtails, Meadow Pipits and Black-headed Gulls feeding on insects emerging from the banks of rotting seaweed, but the stars were definitely the Whimbrel.

These banks of seaweed will be even more important in May, when there will be hundreds of waders refuelling for the next leg of their journeys, with Sanderling and Dunlin on their way to Greenland and some Knot and Turnstone perhaps flying as far as northeast Canada. When the tide is high enough to wash maggots out of the seaweed, you can sometimes see Red-necked Phalaropes along the tide-line, spinning around and picking food off the surface of the water.

Whimbrel migration

We knew that the Whimbrel were on their way because Global Flyway Network had published a map showing the location of ‘Acuno’, a bird wearing a satellite tag that was put on in the Bijagós archipelago of Guinea Bissau. It had been logged west and north of Ireland on the previous evening, already 6000 km and 4 days into its migration. Doing the sums, it seems unlikely that it could have reached Eyrarbakki by the time that we were there but perhaps it flew past soon after, sending signals back to mission control that would confirm arrival**.

** An hour after I published this blog, Acuno was found to have diverted to the Faeroes. It may breed there – as many Whimbrel do – or if could have run out of fuel and landed there, to put on some extra grammes of fat. We’ll see whether it resumes migration.

** Three weeks later, Acuno flew to Iceland. How much will it have been disadvantaged by giving up on direct flight?

Only a minority of Whimbrel fly straight from Africa to Iceland in spring although almost all fly directly south in the late summer. Most individuals spend late April and early May in Ireland, the UK or on the west coast of mainland Europe. Ireland is by far the most important staging area. The individuals on Eyrarbakki beach may have been tired but there could be advantages to being an early bird. See Time to nest again? based on a paper by Morrison et al.

There are several WaderTales blogs about Icelandic Whimbrels:

Whimbrels on the move summarised the movements of Icelandic birds, based on reports of ringed and colour-ringed individuals. In the paper upon which the blog was based (Gunnarsson & Guðmundsson) there was a strong suggestion that birds only stop off in Britain & Ireland on the way north. Geolocator-based research by Alves et al showed that at least some birds were flying straight from Iceland to West Africa and that these sea-crossings could be very rapid.

Migrations to and from Africa were investigated further in a paper by Camilo Carneiro et al that was summarised in Iceland to Africa, non-stop. More recently, papers by the same team have shown that the most consistent point of the annual migration story is departure from Africa and discussed the links between weather and phenology. These two papers have appeared as the WaderTales blogs – Whimbrel: time to leave and A Rhapsody of Whimbrel.

The latest blog about this research is Winter conditions for Whimbrel, based on a paper that assesses the influence of winter conditions on subsequent breeding performance.

Searching for Black-tailed Godwits

We had seen a flock of eight Whimbrel, earlier in the morning, when we were checking fields for colour-ringed Black-tailed Godwits. The Whimbrel were gliding into land, about five km from the coast, before seeming to melt into a patch of rough grassland, bleached after winter frosts.

We did not pay the Whimbrel much attention as, on the other side of the road, there were Black-tailed Godwits probing for worms in silage fields that were already green, after a few days of warmth and the liberal addition of fertilizer. This is our target species during spring trips to Iceland. We have discovered that the arrival time of individuals is remarkably consistent from year to year, which initially seemed surprising, given that migration appears to be getting earlier. There is more about this in Why is spring migration getting earlier? based on a paper in Proceedings of the Royal Society B.

One of the fascinating things about visiting Iceland is that no two years are the same. 2022 has been a dry, warm spring, with northerly winds potentially delaying migration from Britain and Ireland, as mentioned earlier. Early-arriving Black-tailed Godwits that were wearing colour-rings were birds that winter in Portugal and France and migrate via the Netherlands. There was a period of helpful winds for these early birds that fly further but get to Iceland earlier. This strategy is discussed in the blog Overtaking on Migration, based on a paper in Oikos.

Looking forwards

The short Icelandic summer provides fantastic conditions for these waders to raise their chicks, although there are concerns as to how agricultural development, increased forestry and infrastructures will affect these species in the future. In June, in just two months’ time, adults will cross the Atlantic. Black-tailed Godwits head for the British Isles and the west coast of continental Europe and Whimbrel will return to West Africa. By August, the next generation will be preparing for the journey south and we will be here in future springs to monitor their return to Iceland.

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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.

Navigating a vast ocean

The journeys that shorebirds make, as they cross the oceans of the globe, are truly remarkable. Individual birds demonstrate amazing endurance and navigational expertise while in the air for days at a time. Satellite transmitters are providing opportunities to understand how shorebirds refine their flight plans in responses to the wind patterns they encounter. In a paper in Movement Ecology by Jenny Linscott and colleagues, we join the northward flight of Hudsonian Godwits across 7,000 km of the Pacific Ocean and the Gulf of Mexico.

Coping with wind

Conditions encountered en route can dramatically impact the energy that migratory species spend on movement. To complete their journeys across barriers such as oceans, migratory birds need to manage energetic costs by adapting to the wind conditions they encounter. It’s a dynamic environment; no two years are the same and there is often little relationship between local conditions upon departure and what lies ahead, as for instance discussed in the WaderTales blog Rhapsody of Whimbrel.

Migrating Hudsonian Godwits are heading for Alaska

If an individual bird has full knowledge of its location at all times and a fixed destination point in mind, then perhaps it can adjust its airspeed and direction of flight so that it can completely compensate for any lateral drift that is caused by the wind systems en route. Alternatively, it might accept the drift that it experiences or only partly compensate, perhaps gambling that there may be systems ahead that might cause drift in the other direction. It may even over-compensate for the wind patterns it experiences at a particular time, which may be a good idea if there is more of the same weather to come. The authors of the Hudsonian Godwit study explain how these different approaches may integrate to create a movement strategy and summarise what has been found in other studies. Interestingly, it has been suggested that complete compensation (continually adjusting the flight-plan) might not be possible for trans-oceanic flights because there are no landmarks to use as reference points.

Hudsonian Godwits spend the austral summer in coastal Chile and Argentina and migrate northwards in spring, through the midcontinental United States, to breed in subarctic Alaska and Canada. For birds taking off from Chiloé Island in Chile, the first part of the journey takes them across the open Pacific Ocean, the relatively narrow landmass of Central America and the Gulf of Mexico. Godwits making this flight have few or no opportunities to stop, and they traverse several global wind regimes that differ in directionality and strength along the way. The authors predicted that birds would experience drift during their journeys, especially over the featureless open ocean, and increase compensation as they approach North America. Given the vast distances and time spent in flight, it might be expected that the winds experienced during flight should influence the points where individuals cross the coast into North American airspace.

Flock of Hudsonian Godwits on Chiloé Island in Chile – a long way from Alaska

Tracking transoceanic journeys

During the springs of 2019, 2020 and 2021, Jenny Linscott used satellite tracking devices to follow Hudsonian Godwits, as they migrated northward across more than 7,000 km between Chiloé Island, Chile and the northern coast of the Gulf of Mexico.

Prior to migratory departure, two types of solar-powered satellite transmitters were deployed on a total of 54 adults. Technical details are provided in the paper, alongside information as to how the data were filtered and interpreted. This paper focuses on the journey across the Pacific Ocean and the Gulf of Mexico, as birds head towards a relatively narrow refuelling area in North America (the eastern parts of Kansas, Nebraska, South Dakota, and North Dakota).

For any two points along each bird’s route, between Chiloe and the coast of the Gulf of Mexico, it was possible to calculate distance travelled, ground speed, turning angle, and heading. Having removed low-quality data, the team were left with 29 tracks and 689 locations with which to work. They linked each location with the range of possible wind conditions that godwits were likely to be experiencing at the time, depending upon flight height. They then “calculated the total magnitude and mathematical direction of the wind flow for each location at each altitude using vector trigonometry”. Previous studies have shown that migrating waders change altitude in order to find better wind conditions, allowing the team to assign the most likely wind conditions to each godwit location. Please see the paper for details of the modelling and assumption-testing processes.

Results

A total of 24 complete and 5 partial northward journeys were collected from birds migrating north, including repeat tracks from two individuals, which were followed for three years. The sample size was reduced by device failure/malfunctions, presumably some mortality, and by eight birds that oversummered in Argentina (see Teenage Waders and Gap year for sandpipers for information about oversummering shorebirds).  The 25 birds in the study comprised 14 males and 11 females.

• Godwits for which there were complete tracks undertook continuous flights lasting about six days, covering an average of 8,361 km, before making their first stops.
• Godwit ground speeds were best predicted by a strategy in which individuals flew at the altitude offering the most wind support in the preferred direction of movement, but were restricted to altitudes at or below 3000 m. It seems likely that the godwits were mostly flying between 100 m and 750 m above sea level.
• Tracked godwits travelled along paths which showed a close match to a Great Circle line from Chiloé to the North American target area.
• When flight behaviours were analysed, full compensation was the most frequent behaviour, accounting for 41.1% of all observed flight segments. Fewer segments were associated with partial compensation (23.5%), tail winds (8.1%), full drifting (9.4%), or overcompensation (16.0%).
• The prevalence of full compensation remained constant across wind conditions. For example, full compensation was the dominant behaviour under crosswinds to the east (32.0%) and west (45.3%). Full compensation was also prevalent across regions, comprising the largest proportion of behaviours exhibited over the Pacific Ocean (45.1%), while crossing Central America (23.9%), and in the Gulf of Mexico (31.4%).
• There was considerable variation in migration patterns over the Gulf of Mexico and it is suggested that some birds may have been running out of fuel and heading for the nearest land.
• One individual tracked repeatedly over three years completed its crossing of the Gulf of Mexico in central Texas every year. The other bird tracked for three years had no specific point at which it crossed the coast but always ended up in the same spring staging area.

Jenny Linscott and colleagues found little support for their prediction that godwits would tolerate more drift early in their flight and gradually begin to increase compensation as they approached North America. Instead, both fully supported flight (benefiting from tail winds) and full compensation were common soon after leaving Chiloé. Compensation did not increase with distance travelled, was not constrained during flight over open ocean, and did not influence where an individual ultimately crossed over the northern coast of the Gulf of Mexico, at the end of this flight. Instead, the team found a strong preference for full compensation throughout godwit flight paths. Birds ‘knew’ where they were along their route and could judge how to adjust their headings so as to compensate for the drift they were experiencing at any given time. The paper’s Discussion includes more detailed consideration as to how compensation appeared to operate in different parts of the journey and in different wind conditions.

How do shorebirds, flying over the vastness of an ocean ‘know’ where they are, with no island landmarks? Are individuals within migrating flocks picking up on changes in temperature or humidity, that mark passage through broad wind regimes, can they navigate with reference to sun position and stars, are there magnetic cues, or can they interpret surface swell patterns? Perhaps it’s a mixture of several of these skills? It’s pretty amazing!

Learning more

Jenny Linscott and her colleagues found that fully compensating for wind displacement appears to be a critical strategy for Hudsonian Godwits making a long-distance, transoceanic flight. While godwits often followed wind flow in the early stages of this journey, they nonetheless engaged in full compensation more frequently than any other behaviour during the entirety of the flight, across a vast and apparently featureless ocean. These continuous adjustments help to make sure that birds can fly extremely long distances without running out of energy. The team wonder how well future generations will cope with changing wind systems over warmer seas.

The full paper can be found here:

Compensation for wind drift prevails for a shorebird on a long-distance, transoceanic flight. Jennifer A. Linscott, Juan G. Navedo, Sarah J. Clements, Jason P. Loghry, Jorge Ruiz, Bart M. Ballard, Mitch D. Weegman, and Nathan R. Senner. Movement Ecology. https://doi.org/10.1186/s40462-022-00310-z

The Hudsonian Godwits studied here are flying 7,000 km north across the Pacific, but this pales into insignificance when compared to Bar-tailed Godwits that commute between Alaska and New Zealand. The Pacific was once considered a barrier to migration but it is increasingly seen as a conveyor belt. There is an excellent review article by Theunis Piersma and colleagues in Ornithology, the title of which explains its content:

The Pacific as the world’s greatest theater of bird migration: Extreme flights spark questions about physiological capabilities, behavior, and the evolution of migratory pathways. Theunis Piersma, Robert E Gill, Jr, Daniel R Ruthrauff, Christopher G Guglielmo, Jesse R Conklin and Colleen M Handel. Ornithology,  https://doi.org/10.1093/ornithology/ukab086

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

New Bar-tailed Godwit Subspecies

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

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

Limosa lapponica yamalensis

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

How many subspecies?

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

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

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

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

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

What’s different?

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

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

Tracking through space and time

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

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

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

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

Body size and shape

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

Genetics

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

A new subspecies

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

Breeding locations of tagged Bar-tailed Godwits

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

Conservation considerations

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

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

Paper

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

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

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

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

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

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

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

The flock now departing

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

Setting off on migration

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

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

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

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

Tracking Eurasian Curlew

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

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

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

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

Spring migration

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

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

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

Back-mounted GPS tag

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

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

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

Post-breeding migration

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

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

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

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

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

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

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

Migration of juveniles

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

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

Paper

The nutrient-rich mud of Ile Madame

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

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

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

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

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

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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.

Flying high with Great Snipe

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

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

Great Snipe

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

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

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

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

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

Great Snipe watches on as team members set a mist net

Flying high

Processing the catch

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

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

Methods

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

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

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

How high?

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

Breeding habitat in Sweden

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

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

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

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

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

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

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

Discussing the results

Great Snipe with a datalogger

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

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

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

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

Paper

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

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

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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.