Population fitness has a concave relationship with migration distance in Sanderlings

Abstract

In Focus: Reneerkens, J., Versluijs, T. S. L., Piersma, T., Alves, J. A., Boorman, M., Corse, C., … Lok, T. (2020). Low fitness at low latitudes: wintering in the tropics increases migratory delays and mortality rates in an Arctic breeding shorebird. Journal of Animal Ecology, 89, 691–703. A central question in migratory ecology has been to understand the fitness consequences of individual variation in migration distance among different species and populations. Reneerkens et al. (2020) investigated the demographic consequences of long‐distance migration for Sanderlings Calidris alba, an Arctic‐breeding species of sandpiper. Their study population has a remarkable geographic distribution with a breeding range that is concentrated in northeast Greenland and Ellesmere Island, Canada but a nonbreeding range that extends across 85° of latitude from Scotland to Namibia. The authors report on unexpected patterns of latitudinal variation in three demographic parameters: timing of passage on northward migration, probability of juvenile migration and apparent survival of adults. Sanderlings travelling 1,800–2,800 km to settle at north temperate sites during the nonbreeding season had earlier passage dates, and also higher probabilities of migration and apparent survival. In contrast, birds travelling 6,000–7,800 km to equatorial sites experienced later passage dates, delayed maturity and lower apparent survival. However, if Sanderlings migrated even farther and flew over 11,000 km to nonbreeding sites in Namibia, then their performance was restored to early passage dates and higher survival. Movement tracks from birds tagged with geolocators showed that birds wintering in Namibia make nonstop flights of 7,500 km that bypass West Africa during northward migration. Thus, all lines of evidence suggest that Sanderlings face adversity when spending the nonbreeding season at equatorial latitudes. Moreover, the central finding that components of fitness can have nonlinear relationships with migration distance is a novel discovery that leads to many additional questions. The new findings have broader implications for theoretical models of migration, and for understanding how different patterns of movements may arise or be maintained in migratory species.

Migration tactics and connectivity of a Nearctic–Neotropical migratory shorebird

“…We also found one SY female that conducted an incomplete migration, which, to our knowledge, is the first documented occurrence of this for shorebirds. Migratory distance has been found to affect migration tactics in Calidrine sandpipers (Sandercock, 2020), and we expected western breeders would have shorter durations. We did find they arrived and departed at earlier dates than central and eastern breeders, but this pattern was not statistically significant (Nilsson et al, 2013;Piersma, 2007).…”

“…Younger birds in their first pre‐breeding migration often migrate more slowly than adults, choose suboptimal stopover habitat and may even fail to complete migrations, presumably due to the lack of knowledge in the migratory routes (Gratto, 1988; Hockey et al, 1998). Additionally, distance to the breeding location can also affect timing during northbound migration, particularly if differences exist among breeding habitat location and quality (Gunnarsson et al, 2006; Sandercock, 2020).…”

“…Morphometrics, primarily bill length, allowed us to estimate approximate breeding region (Gratto‐Trevor et al, 2012) and, for adults, the feather isotope ratios provided some information about wintering location (Appendix S1). Because male SESA establish breeding territories, second‐year birds are less experienced and the western breeding individuals have a longer distance to migrate, we predicted that males, after second‐year birds and western breeders would have shorter stopover durations (Gratto, 1988; Sandercock, 2020). Further, since the mid‐Continental wetlands in North America used by SESA are limited and unreliable during spring migration (Gillespie & Fontaine, 2017), we predicted that SESA would follow the expectation rule of migration (Alerstam, 2011), remaining longer at the higher quality site to take advantage of habitat after crossing the GOM (Alerstam, 2011).…”

“…Morphometrics, primarily bill length, allowed us to estimate approximate breeding region (Gratto-Trevor et al, 2012) and, for adults, the feather isotope ratios provided some information about wintering location (Appendix S1). Because male SESA establish breeding territories, second-year birds are less experienced and the western breeding individuals have a longer distance to migrate, we predicted that males, after second-year birds and western breeders would have shorter stopover durations (Gratto, 1988;Sandercock, 2020).…”

Low juvenile survival threatens the Black Stork Ciconia nigra in northern Europe

“…Variation in demographic parameters, which is frequently age specific, guides the management of wildlife populations (Sandercock 2006). Among demographic parameters, survival is often thought to have the greatest impact on population dynamics, especially among long-lived species (Doherty et al 2004; Schorcht et al 2009; Sandercock 2020). However, in wild populations, separating survival from dispersal is challenging because it is often impossible to distinguish between the death and emigration of an animal.…”