Sex-specific timing of autumn migration in birds: the role of sexual size dimorphism, migration distance and differences in breeding investment

Abstrakti

Multiple studies have investigated differential migration of sexes during spring migration, while such differences during autumn migration are poorly studied.We tested several functional hypotheses explaining differences in autumnmigration dates between sex and age classes and whether these patterns vary between short- and long-distance migrants (SDMs andLDMs, respectively).We used data of ringed birds from the Hanko and Lågskär Bird Observatory, Finland, North Europe. Altogether data for c. 200,000 ringed birds including 14 passerine species were used. Protogyny,with femalesmigrating earlier than males, was common among young birds, and this difference was clearer in LDMs than inSDMs.However, in adults protogyny was not found, whereas protandy,malesmigrating earlier than females,was found in two species. Furthermore, species-specific sexual size dimorphism, SSD, was significantly connected with the time differences in migration between the sexes in SDMs, but not in LDMs. These results suggest thatmultiple factors are likely affecting differential timing of autumnmigration in birds. It can be beneficial for males, especially young birds, to spend additional time at the breeding grounds to prospect for future nesting sites. The connection between SSD and autumn migration dates in SDM could be linked with the pattern where larger sized individuals can winter closer to the breeding grounds. In addition, later migration dates of adult females compared to adultmales could suggest that larger reproductive investment by adult females on breeding may delay their autumn migration.