Sex-specific differences in spring migration timing of Song Thrush Turdus philomelos at the Baltic coast in relation to temperatures on the wintering grounds

Abstract

Environmental factors influencing the timing of bird migration frequently affect age and sex groups differently. We applied discriminant functions to determine the sex of adult Song Thrushes ringed during spring migration in 1968–2019 at Hel on the Polish Baltic coast. We aimed to determine any differences between the sexes in the timing of spring migration and changes in protandry, and to identify any relationships between migration timing and temperatures at the wintering grounds. We analysed relationships between the date of migration and the year and monthly mean minimum temperatures in December–February by sex, using multiple quantile regression for q10, q50 and q90 of spring passage. On average males migrated six days before females. Spring protandry increased over 1968–2019 and with warm winters. The sexes responded differently to winter temperatures, likely because of the species’ mating strategy. For males the warmer the winter, especially in February, the earlier spring migration began and the later it ended, prolonging its duration. The influence of February’s temperature on the beginning of males’ migration is likely explained by selective pressure for their early arrival to establish territories. The later end of their arrival with a warm winter might reflect more low-quality males surviving, and migrating later than high-quality males, than with cold winters. For females December and January temperatures had no effect on the beginning and the end of spring migration; but the warmer the February, the earlier migration started and the later it ended. Females probably benefitted from improved conditions in a warm February in two ways: early-arriving females maximized their breeding success with a chance to raise a second brood, and late-arriving females maximized their chances of surviving migration by prolonging fuelling and stopovers, which extended females’ arrivals.