Oversummering juvenile and adult Semipalmated sandpipers in Perú gain enough survival to compensate for foregone breeding opportunity

Abstract

Background Age at maturity and the timing of first breeding are important life history traits. Most small shorebird species mature and breed as ‘yearlings’, but have lower reproductive success than adults. In some species, yearlings may defer northward migration and remain in non-breeding regions (‘oversummering’) until they reach 2 years of age. Some adults also oversummer. Oversummering would be favoured by natural selection if survival were as a result raised sufficiently to compensate for the missed breeding opportunity. Several thousand Semipalmated Sandpipers (Calidris pusilla) spend the non-breeding period at Paracas, Perú, including individuals with long bills (likely from eastern Arctic breeding populations ~ 8000 km distant) and short bills (likely from western Arctic breeding populations, up to 11,000 km distant), with short-billed birds more likely to oversummer. We tested the prediction that oversummering birds have higher survival than migrants, and that the magnitude of this higher survival for oversummering birds is enough to compensate for their lost breeding season. Methods We used a Multi-State Mark-Recapture model based on 5 years of encounter data (n = 1963 marked birds, and 3229 resightings) obtained year-round at Paracas, Perú, to estimate seasonal (i.e. breeding and non-breeding) survivorship for migrant and oversummering birds. We calculated the magnitude of the oversummering survival advantage required to compensate, for both yearlings and adults, based on published measures of annual survival and reproductive success. Using bill length as a proxy for migration distance, we investigated whether migratory survival is distance-dependent. Results We estimate that 28% of yearlings and 19% of adults oversummer. Survival is higher for oversummering birds than for migrants, and the oversummering survival advantage is greater for adults (0.215) than for yearlings (0.140). The theoretical thresholds predicted by the size of the missed reproductive opportunity are 0.240 for adults and 0.134 for yearlings. Migratory survival decreases and the oversummering rate increases with migration distance, as assessed by culmen length. Conclusions Our results support the life history hypothesis that oversummering raises survival enough to compensate for the loss of a breeding opportunity. Greater migration distance lowers survival and increases the probability of oversummering.

First tracking of declining Caspian terns Hydroprogne caspia breeding in the Baltic Sea reveals high migratory dispersion and disjunct annual ranges as obstacles to effective conservation

“…In the case of our study population, there is variation between subadults in the decision to start a northwards journey that may depend on individual condition (Tavera et al 2020) and be related to learning and development (Sergio et al 2014). However, the partial return migration of some subadults to common stopover sites, and in two cases back to the Baltic Sea, shows that Caspian terns at this age are capable of migrating successfully.…”

“…In other bird species, such as shorebirds, it seems that this phenomenon may be selected for if the costs of migration outweigh the benefits of reproduction when breeding success is low in the early years of life (Summers et al 1995, Hockey et al 1998). In the case of our study population, there is variation between subadults in the decision to start a northwards journey that may depend on individual condition (Tavera et al 2020) and be related to learning and development (Sergio et al 2014). However, the partial return migration of some subadults to common stopover sites, and in two cases back to the Baltic Sea, shows that Caspian terns at this age are capable of migrating successfully.…”

Danger, risk and anti‐predator behavior in the life history of long‐distance migratory sandpipers

“…To be advantageous, over‐summering survival must therefore exceed that of migration by a margin at least large enough to compensate in fitness terms for the foregone breeding season. The required minimum (termed s* ) is equal to the proportion of expected lifetime reproductive success necessarily foregone (explained in Appendix 3 of Tavera et al 2020). Estimates of s* and of the survival gained by over‐summering (termed s ) at Paracas are given in Tavera et al (2020) (for semipalmated sandpipers; Appendix 3) and Tavera (2020) (for western sandpipers; Appendix C).…”

“…Survival rates of these species were recently estimated in two independent studies (Table 1). The study by Tavera (2020) and Tavera et al (2020) took place at Paracas, a non‐breeding site in Perú, where both species are present year‐round. Estimates could be made of annual (October–September) as well as ‘seasonal' (‘winter' October–March; ‘summer' April–September) survival rates, for both migrants and over‐summering individuals.…”

“…The annual survival of the 19% of adults that over‐summer is calculated as the product of summer non‐migrant (0.894) and winter survival (0.904). 2 Based on the ‘residency' estimate October–March, reported by Tavera (2020). 3 Estimate based on yearlings.…”