Testes play a role in termination but not in initiation of the spring migration in the night-migratory blackheaded bunting

Day length regulates the development of seasonal phenotypes linked with both the spring migration (e.g., premigratory body fattening and intense nighttime restlessness, called Zugunruhe, in captive birds) and reproduction (e.g., gonadal growth and maturation). The apparent overlap in these photoinduced seasonal phenotypes could be taken to suggest that they are causally linked. The present study investigates this, using the night-migratory blackheaded bunting (Emberiza melanocephala). We continuously monitored activity of male buntings exposed for 19 weeks to short (8 h light : 16 h darkness; 8L : 16D) and long (16L : 8D) photoperiods. Long, but not short, days induced the spring migratory phenotype. Another experiment investigated the role of testes in spring migration by comparing the development of Zugunruhe between intact and castrated buntings held on natural day lengths, at Meerut (India, 29°01′N, 77°45′E) during the period from March to October, thus covering the times of both the spring and autumn migrations. Testes were not involved in induction of the migratory phenotype but probably influenced the end of the migratory season, since castrates delayed the termination of the Zugunruhe.