Nocturnal melatonin levels decode daily light environment and reflect seasonal states in night-migratory blackheaded bunting (Emberiza melanocephala)

We proposed two perhaps overlapping hypotheses.

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.

Temperature alters the photoperiodically controlled phenologies linked with migration and reproduction in a night-migratory songbird

We investigated the effects of temperature on photoperiodic induction of the phenologies linked with migration (body fattening and premigratory night-time restlessness, Zugunruhe ) and reproduction (testicular maturation) in the migratory blackheaded bunting. Birds were exposed for four weeks to near-threshold photoperiods required to induce testicular growth (11.5 L:12.5 D and 12 L:12 D) or for 18 weeks to a long photoperiod (13 L:11 D) at 22°C or 27°C (low) and 35°C or 40°C (high) temperatures. A significant body fattening and half-maximal testicular growth occurred in birds under the 12 L, but not under the 11.5 L photoperiod. Further, one of six birds in both temperature groups on 11.5 L, and four and two of six birds, respectively, in low- and high-temperature groups on 12 L showed the Zugunruhe . Buntings on 13 L in both temperature groups showed complete growth-regression cycles in body fattening, Zugunruhe and testis maturation. In birds on 13 L, high temperature attenuated activity levels, delayed onset of Zugunruhe by about 12 days, reduced body fattening and slowed testicular maturation. The effect of temperature seems to be on the rate of photoperiodic induction rather than on the critical day length. It is suggested that a change in temperature could alter the timing of the development of phenologies linked with seasonal migration and reproduction in migratory songbirds.