With the approach of spring in the Northern Hemisphere, waders (shorebirds) that have spent the previous few months on the shores of Africa, the Americas, Asia, Australasia, Europe and oceanic islands need to decide whether to fly north. Those that depart leave behind flocks of young birds, together with adults that ‘choose’ not to migrate north and attempt to breed. In a paper in Movement Ecology, Eveling Tavera and colleagues investigate how the decisions made by individual Semipalmated Sandpipers that spend the non-breeding season in Paracas (Peru) are likely to affect their chances of survival.
The Semipalmated Sandpiper is one of the smallest of the world’s waders. The species is designated as near-threatened by BirdLife International and IUCN, despite its large breeding range. In their assessment, they draw attention to declining numbers, potentially linked to reduced food supplies in staging areas and to changes to arctic breeding habitat. Hunting pressure in some parts of the wintering range (especially the eastern seaboard of South America and in the Caribbean) may particularly affect Semipalmated Sandpipers that breed in eastern Canada.
Back in the 1970s, when I spent three summers in James Bay (northern Canada), I remember catching skinny juvenile Semipalmated Sandpipers that weighed under 20 g, which is way smaller than a House Sparrow (24.2 g – 30.7 g: BTO BirdFacts). We also caught fat, 40 g adults that were about to leave and head to countries such as Suriname in South America, perhaps stopping off in the Bay of Fundy on their way south. In just a few months, it would be time to fly north again and I guess we assumed that any bird that was alive would at least try to make the return journey. After all, these are small shorebirds and most small shorebirds breed in their first year.
The distribution of Semipalmated Sandpipers during the non-breeding season covers a broad range of latitudes, from about 25°N (southern Florida) to 23°S (northern Chile and southern Brazil). Some of the birds flying to Arctic Canada, Alaska and Northeast Russia are travelling a lot further than others. A bird setting out on a return journey to northern Quebec from Peru will need to find more resources to fuel its journey than one flying from the Caribbean. The additional fattening requirement could potentially affect the ability of individuals to make the return migration.
In their global review of annual survival rates of shorebirds, Verónica Méndez and colleagues were able to include 56 species from around the globe. Unsurprisingly, small waders tend not to live as long as larger species. Combining the various studies of Semipalmated Sandpiper, they estimated an annual survival rate for adults of 0.61 (see Measuring shorebird survival). This means that, on average, there’s a 39% chance of an individual dying between one breeding season and the next.
We know that waders don’t always breed every year, as discussed in Teenage Waders. Small waders have lower life expectancy than large waders (Waders are long-lived birds) so missing out on a breeding season makes much more of a difference to a small sandpiper than to a Knot (estimated survival rate 0.8 or 20% chance of dying within the next twelve months) or an Oystercatcher (0.89; 11%).
As Tavera et al point out in the introduction to their paper, although most small shorebirds attempt to breed in their first year of life they may have lower breeding success than older individuals. In Semipalmated Sandpipers, young birds start nesting later in the spring, lay smaller and fewer eggs, and produce fewer chicks (references in paper). When comparing potential reproductive output, a bird that does not migrate north in its first year is probably missing out less than an adult that ‘takes a year off’.
Semipalmated Sandpipers in Peru
The results presented in the Tavera et al Movement Ecology paper come from a long-term Semipalmated Sandpiper colour-ringing study in the Paracas National Reserve, 250 km south of Lima, in Peru. Over a period of five years, 1963 birds were caught in mist nets and individually marked, producing 3229 resightings. Only eight of the marked birds has been seen during the nesting season, all in Alaska and western Canada. This paper builds on a previous paper: Effects of migration distance on life history strategies of Western and Semipalmated sandpipers in Perú.
The key findings in the study are:
- 28% of first-year Semipalmated Sandpipers remain at Paracas, instead of heading north
- 19% of adults remain in Paracas instead of migrating.
- The apparent annual survival rate of first-years that head north is 0.555, compared to 0.671 for first-years that stay at Paracas.
- For adults, apparent annual survival rate of those that migrate is 0.614, compared to 0.808 for those that stay.
The fact that 28% of youngsters in this particular population of Semipalmated Sandpipers don’t migrate north in the first year was unexpected, given what had been learnt from previous studies of the species, but the fact that one in five adults ‘choose’ not to breed in any year is probably more surprising. In a species with a declining population, these missed opportunities to boost the population appear concerning.
Short and long bills
Several thousand Semipalmated Sandpipers spend the non-breeding period at Paracas, including individuals with long bills (which are thought to fly there from eastern Arctic breeding populations, about 8,000 km away) and short bills (likely from western Arctic breeding populations, up to 11,000 km distant). There is more about these morphometric differences in this paper by Cheri Gratto-Trevor et al in Waterbirds.
It is not possible to assign individual Semipalmated Sandpipers to a particular population, especially as there are also sex-based differences in bill length, and gender could not be determined in the hand. However, there is a trend for longer-billed birds to be from the east. The eight colour-marked birds from Paracas that have been seen in the western part of the breeding range were all short-billed or intermediate-billed birds.
- Larger-billed juveniles appear more likely to migrate than smaller-billed juveniles, suggesting that Semipalmated Sandpipers that have travelled less far are more likely to breed in the first year.
- Resightings of flagged birds suggest that the survival of first-year birds that fly to the Arctic in their first spring is markedly lower in small-billed birds than in long-billed birds.
The cost of migration seems particularly high in small-billed, first-year Semipalmated Sandpipers. These birds tend to be longer-distance migrants, from the western end of the breeding range.
Modelling the costs and advantages of migration
Eveling Tavera and her colleagues have found that survival is higher for Semipalmated Sandpipers that stay in Paracas than for migrants. During the period April to September, the apparent survival rate for first-year birds and adult birds is much higher for birds that do not migrate. The survival advantage acquired by non-migrants is significantly greater for adults (0.215) than for yearlings (0.140).
By failing to migrate, individual Semipalmated Sandpipers miss out on a breeding opportunity. Does a bird that lives longer but does not breed in every potential year produce more youngsters than a bird that lives a little less long but breeds every year? In an appendix to the paper, the research team calculate whether the survival advantage for birds that stay in Paracas is high enough, in fitness terms, to compensate for the loss of potential youngsters. They used values for annual survival, mean clutch size and hatching success from studies by Weisser et al and Gratto et al, for adults and first-year birds, to produce figures for Lifetime Reproductive Success (LRS). The predicted survival advantage of non-migrating adults (0.240) is higher than that of first-years (0.134) because they forego a larger average potential reproductive output than do inexperienced birds.
Using these calculations:
- On average, a first-year migrant will have the same LRS as a bird that does not migrate if its survival in the first year is no more than 0.134 lower. This is close to the measured value of 0.140 for the Paracas birds.
- On average, an adult migrant will have the same LRS as a bird that does not migrate if its survival in that year of life is no more than 0.240 lower. This is close to the measured value of 0.215 for the Paracas birds.
Based on the modelling of LRS and survival rates, the team conclude that ‘juvenile and adult birds staying at Paracas compensate for the loss of a breeding opportunity with higher survivorship than migrant birds’.
Different non-breeding populations
The Semipalmated Sandpipers that breed across North America and the far eastern tip of Russia travel to a wide range of destinations in the autumn, with birds mixing in the non-breeding season. The Paracas site is a long way from the species’ breeding grounds and the results may suggest that migration is constrained by distance. However, there could be other processes at play, especially if food supplies are limited at the time when individuals need to fatten up for migration. Jeroen Reneerkens et al showed that Greenlandic Sanderling that spend the winter near the equator (Mauritania & Ghana) had lower apparent survival rates than birds that travelled much further south (Pretoria). They were also less likely to breed in their first year than birds spending the breeding season in other locations, and arrived on their breeding territories late in the season. Migration distance is not always a problem, it transpires, as long as there are refuelling possibilities on the way north, as described in Travel advice for Sanderling. This is also discussed in Overtaking on Migration, a Black-tailed Godwit blog based upon a paper by Alves et al.
Much of the statutory protection for migrant waders is based upon sites used in the non-breeding season. Eveling Tavera has shown that Paracas is important for the whole year. Are there other sites that hold 20% or more of their flocks of Semipalmated Sandpipers for the whole year? What are the conservation implications for this species, which is already designated as ‘near-threatened’? Are we doing enough to look after pre-breeding (and non-breeding) flocks? This issue is discussed further in Teenage Waders, built around a Hudsonian Godwit paper by Juan Navedo & Jorge Ruiz.
An individual Semipalmated Sandpiper does not, of course, have the experience or capacity to enable calculation of differential survival rates. The decision to migrate north will be based upon circumstances and the bird’s condition. Effectively, it is trading off the fitness benefit of higher survival against the fitness cost of a foregone breeding opportunity – but it does not know that that is what it is doing. What is interesting, in this study, is that the cost-benefit analysis so closely matches what is found using field data from this particular site. The survival rates and percentage of non-breeding Semipalmated Sandpipers in Suriname or at another site in Peru may well be completely different but could still balance out. There is so much more to learn about shorebird migration.
Oversummering juvenile and adult Semipalmated sandpipers in Perú gain enough survival to compensate for foregone breeding opportunity Eveling A. Tavera, Glenn E. Stauffer , David B. Lank and Ronald C. Ydenberg. Movement Ecology 8,42. https://doi.org/10.1186/s40462-020-00226-6
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.