Endocrine Correlates of Autumnal Behavior in Sedentary and Migratory Individuals of a Partially Migratory Population of the European Blackbird (Turdus merula)

Abstract

Abstract Plasma levels of luteinizing hormone (LH), testosterone, 5α-dihydrotestosterone (5α-DHT), estradiol-17β, and corticosterone were measured in free-living and captive European Blackbirds (Turdus merula) during autumn. Free-living birds were designated as sedentary or migratory according to whether they remained in or disappeared from the study area during the winter season. Captive birds that increased body weight, deposited fat, and developed Zugunruhe in autumn were designated as migratory, whereas those that failed to do so were regarded as sedentary. The results do not support the hypothesis that plasma levels of gonadal hormones are elevated in sedentary birds, thus inhibiting autumnal migration. Plasma levels of LH and of testosterone were low in both adult and first-year free-living males, although testosterone levels were higher in migratory than in sedentary free-living first-year males. In captivity, however, testosterone levels were not different between first-year sedentary and migratory males. In both free-living and captive first-year females levels of 5α-DHT were higher in migratory than in sedentary individuals. Our analyses suggest that the sedentary and migratory habit may be a consequence of different reactions of genetically different individuals to a slightly increased secretion of gonadal hormones in autumn. The variable plasma levels of corticosterone suggest that this hormone is not directly involved in the initiation of migratory behavior, although circulating levels of corticosterone may be influenced by energetic demands during migration.