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 Ecologyhttps://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


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.

Teenage waders

Hudsonian Godwit in breeding plumage

A paper by Juan Navedo & Jorge Ruiz, focusing on Hudsonian Godwits, raises important points about the conservation of the world’s larger shorebirds.

Many curlews and godwits don’t breed in their first year but what do they do instead and how quickly do individuals recruit into the breeding population? These answers have direct implications for the conservation of these species, numbers of which are declining in most cases.

Hudsonian Godwits

Hudsonian Godwits breed in Alaska and Canada and spend the non-breeding season in Chile and Argentina. There are three well-separated breeding populations; in south-central and western Alaska, along the northwest coast of Canada (Mackenzie and Anderson river deltas) and within the Hudson and James Bay region of northern Canada. These are indicated in the figure alongside, based on the map from BirdLife International’s datazone.

On migration, Hudsonian Godwits do not use coastal areas, which led to the theory that their journeys north and south might be made without a break. Satellite tracking has revealed that staging areas are continental rather than coastal. The latest research shows that birds wintering in Chile stop off in the prairies of North America on the way north. On the return journey, there are key refuelling areas in Saskatchewan (Canada) and in continental wetlands between Colombia and Argentina. Map below is from a paper from Senner et al.

Satellite tracking of two birds from the Mackenzie Delta breeding population, on Canada’s Arctic coast, revealed a 1500 mile two-day ‘hop’ to Hudson Bay, a long refuelling and moulting period and then a 4 or 5 day direct flight to South America. These are very impressive migrants!

As discussed in Why are we losing our large waders?, the Hudsonian Godwit is still considered to be of ‘least concern’ on the IUCN/BirdLife list. The fact that the species breeds over a broad sweep of Alaska and Canada, even if in discrete areas, and that the population, estimated to be 77,000 individuals (Andres et al 2012), is well over the 10,000 cut-off for threat consideration, means that the species is not yet designated as being of international concern. In Canada, Hudsonian Godwit was added to the ‘threatened species’ list in 2019, as a consequence of reduced breeding success and a major decline in the Canadian breeding population. The latest population estimate in the COSEWIC report is that there are just 41,000 mature individuals (24,300 in the Hudson Bay Lowlands, 800 in the Mackenzie Delta, and 15,750 in Alaska).

Non-breeding youngsters

Releasing a satellite-tagged Hudsonian Godwit

When Hudsonian Godwits depart from the coasts of Chile and Argentina, on their way to Alaska and Canada, they leave behind young birds that will not breed in their first year – and possibly even the second or third. These sub-adults are the future of Hudsonian Godwit conservation. In a declining population, it is important that as many as possible of these youngsters will reach maturity and breed successfully.

Juan Navedo, Jorge Ruiz and colleagues from Universidad Austral de Chile, have been studying the ecology and migratory behaviour of Hudsonian Godwits that winter in Chiloé Island (c. 32°S, Chile) as summarised here. One of the unexpected outcomes of this work is a 2020 paper in Global Ecology and Conservation, entitled Oversummering in the southern hemisphere by long-distance migratory shorebirds calls for reappraisal of wetland conservation policies. In it, they followed the movements of a small number of satellite-tagged adults and discovered significant flocks of non-breeding birds. Their great detective work has clear implications for the potential recovery of Hudsonian Godwit populations and wider consequences for other species of shorebirds that don’t breed in their early year or years of life.

Spring departure

Hudsonian Godwits in Chile are being tracked to improve understanding of the connectivity between their discrete breeding areas and their wintering areas. Tracing their journeys also helps to establish the refuelling areas that are used during northwards and southwards migration.

When it was time to depart from Chiloé Island in spring, most adult Hudsonian Godwits spent a week flying north non-stop over the Pacific, crossed Mexico and staged in the plains of North America but, in 2017, three tagged birds unexpectedly headed northeast to the Pampa wetlands of Argentina. These birds all remained in the Pampa area for five or more months. Their tags provided positions every thirty minutes, supplying the scientists with the information they needed to identify key ‘oversummering’ areas for the species. By focusing on groups of locations for two of the tagged birds, it was possible to identify a 28,000 km2 area that seemed to be of particular importance. Was this where young Hudsonian Godwits (not wearing tags) spent the same period?

The paper explains how the team searched the vast area of permanent and ephemeral wetlands in a systematic way, allowing estimates to be made of the number of shorebirds of a range of species that spend time in this area, instead of migrating north to breed. Hudsonian Godwits were found in four (out of 44) wetlands in 2018 and three in 2019, making a total of 366 and 746 individuals in the two years.

Looking for Hudsonian Godwits in the Pampa Wetlands of Argentina

In their three-day surveys, the team was only able to survey a small number of potential feeding sites within the vast area. On the assumption that Hudsonian Godwits mature at the same rate as other large, long-distance migrant waders, such as limosa Black-tailed Godwits and baueri Bar-tailed Godwits, Juan Navedo and Jorge Ruiz suggest that nearly half of first-year, a quarter of second-year and 15% of third-year Hudsonian Godwits may be using these continental wetlands during a crucial stage of their lives. Numbers of Lesser Yellowlegs and Greater Yellowlegs that they found in the same habitats were also of conservation significance, and the Pampa wetlands are also a ‘summering’ area for Buff-breasted Sandpiper and Upland Sandpiper.

Conservation implications for Hudsonian Godwits

Young Hudsonian Godwits will hopefully breed in Alaska in later years

The paper by Juan Navedo & Jorge Ruiz clearly demonstrates the significance of the Pampa wetlands for waders that were raised in the Northern Hemisphere but which don’t return there until their second, third or even fourth potential breeding season. What is it about these wetlands that is so important to young Hudsonian Godwits, how vulnerable are they and what other inland areas are being used by flocks of young birds? The authors point out that the pampas habitats of Argentina are changing. The Pampa wetlands are being turned into vast swaths of agricultural land, much of which is being planted with herbicide-resistant transgenic soybean. Globally, it is estimated that 77% of soybean is grown to supply animal feed. This is a thirsty crop that requires irrigation, sucking water out of the remaining wetlands. These grassland habitats are subject to a Memorandum of Understanding on the Conservation of Southern South American Migratory Grassland Bird Species and Their Habitats.

Flagged Hudsonian Godwit

The presence of some adult birds in the Pampa wetlands flocks is not surprising. As discussed and referenced in the paper, individuals that migrate long distances and that are on tight schedules may need to skip the occasional breeding season if they do not have sufficient fat reserves to head north on schedule. When studying tracked individuals of migratory shorebirds that undertake long, non-stop flights, it has been shown that birds occasionally abort their journeys if they encounter adverse weather conditions. We know that godwits of other species can live for thirty years or more so there is always next year.

Wider implications

The Hudsonian Godwit paper is not just about one species. It asks important questions about the conservation of waders and other families of birds that do not breed in their first year. Globally, have we identified the most important shorebird sites, can we protect them from development and are some sites more important than others? Do we pay these sites enough attention in the breeding season, when the large swirling flocks have departed, leaving much smaller aggregations of non-breeding birds sparsely distributesd throughout flyways? Protecting these small oversummering flocks is investing in the future of threatened wader species.

Site protection: This paper has highlighted an important issue. What do young curlews, godwits and other large and medium-sized waders do in the ‘teenage’ years – that important period between being a juvenile and being a breeding adult? Where are they? Are the sites protected? Do birds simply stay in the areas in which they settle in August, September or October, after they have flown south from the breeding grounds? The Pampa wetlands of Argentina are unlikely to be the only non-coastal areas that are used by young waders; what other sites are we missing? For Hudsonian Godwits, perhaps there needs to be a broader definition of key shorebird areas when considering candidate sites that need to be protected as part of the excellent Western Hemisphere Shorebird Reserve Network? More broadly, the authors propose that conservation agreements regarding wetlands of international concern should include a specific criterion for oversummering areas.

Figure illustrating delayed maturity of Hudsonian Godwits (from Navedo & Ruiz)

Youth clubs: Young waders recruit to the breeding population at different ages. If the typical life expectancy of a godwit is ten years then a bird that flies north in its second summer will, on average, produce 12% more chicks in its life-time than one that does not breed until a year later. Hudsonian Godwits not only need to survive in these Pampa youth clubs, they also need to thrive.

Within the same species, time of first breeding can be influenced by the non-breeding site than an individual happens to use. In their fascinating paper about Sanderling migration, summarised in the WaderTales blog Travel advice for Sanderling, Jeroen Reneerkens and colleagues showed large difference in the proportion of young birds that travelled north in the first summer. Most Sanderling that spend the winter in England and Portugal fly to Greenland in spring but only a tiny proportion of birds wintering in Ghana have this extra breeding attempt. They link these differences to site quality.

Eurasian Curlew and a young Black-tailed Godwit

Disturbance: When the number of Eurasian Curlew on a British or Irish estuary drops rapidly, in March and April, and when adult Far Eastern Curlew leave Australia to head for eastern Russia, do we pay enough attention to the small number of birds that remain?

In the northern hemisphere, in particular, the summer months of May through to July bring increased disturbance pressure to beaches and to estuaries, at a time when non-breeding waders are trying to find the resources they need to undertake their primary moult. Perhaps more thought needs to be given to zoning recreational activities in areas which are internationally designated as conservation areas? A May count of 27 Eurasian Curlew on the Exe Estuary in Devon may seem trivial, when compared to 849 in August (Wetland Bird Survey 2018), but these birds represent the future.

In summary

As Juan Navedo & Jorge Ruiz write in the abstract of their paper:

Given their delayed maturity, many long-distance migratory shorebirds may spend large portions of their lives in previously undocumented wetlands, while deferring migration. These unrecognized oversummering habitats fall outside the scope of today’s conservation efforts for Hudsonian Godwits, because they are not spatially nested within the non-breeding grounds, an issue to be studied for other shorebirds.

We are seeing rapid declines of many of the Numeniini family (see this blog about curlews, godwits and Upland Sandpiper) and other slow-maturing shorebird species, and ‘teenage birds’ deserve more attention. We have to identify the areas used by immature birds as quickly as possible, before productivity is so low that we cannot find them.

Oversummering in the southern hemisphere by long-distance migratory shorebirds calls for reappraisal of wetland conservation policies. Juan G. Navedo & Jorge Ruiz. Global Ecology and Conservation doi.org/10.1016/j.gecco.2020.e01189

Wintering Hudsonian Godwits on Chiloé Island, Chile.

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.