Experience and survival in migratory European Robins Erithacus rubecula and Song Thrushes Turdus philomelos negotiating the Baltic Sea

Abstract

ABSTRACTCapsuleThere were survival costs for adult European Robins Erithacus rubecula grounded at coastal sites following departure, but not for young Robins or Song Thrushes Turdus philomelos.AimsTo compare survival costs of crossing the Baltic Sea in autumn in adults and young of two passerines: the smaller European Robin and the slightly larger Song Thrush.MethodsWe calculated the mortality of experienced adults and those embarking on their first migration, modelling survival in a capture-mark-recapture framework using ringing records from three bird observatories on the periphery of the Baltic Sea. We tested the subsequent age- and species-dependent 12-month interval survival of autumn migrants caught before, during and after the barrier.ResultsSurvival did not differ in relation to the capture site, however, results revealed species-specific post-capture survival. Adult European Robins encountering an ecological barrier on autumn migration had lower survival probability than that of immature birds, and less than a third of adults at following intervals. Yet, in Song Thrushes, adult survival estimates were similar over all time intervals, and estimates for both age classes were comparable to those derived from breeding sites.ConclusionExperienced individuals of the smaller species are likely to become grounded at the periphery of the ecological barrier as a result of physiologically limiting conditions. Species- and age-dependent survival also highlights potential inherent biases of sampling birds at coastal bird observatories.