Melatonin regulation of migratory behaviour and physiology

Migration is a critical life history stage for a large number of bird species. This event involves major physiological changes in nearly every system including hormonal systems. There is an extensive (and sometimes contrasting) literature on the role played by pituitary, gonadal, thyroid, and adrenal hormones in a range of migration-related phenomena such as fat deposition, metabolic adaptations, orientation, and migratory disposition. Little attention has been given so far to the involvement of the pineal hormone melatonin in migratory physiology and behaviour. In most bird species the pineal gland contains an autonomous circadian oscillator that is responsible for the rhythmic release of melatonin, and melatonin is a major component of the avian circadian system. In birds the duration of the night melatonin peak reflects the photoperiod, however, in contrast to what is seen in mammals, melatonin does not appear to be important for the synchronization of avian annual cycles. The crucial link between melatonin and migration becomes evident if we consider that a large proportion of birds are nocturnal migrants, that is they are normally active during the day but migrate mainly or exclusively at night. Hence, in these species, migration is associated with a major switch in the diel pattern of activity, with a supposed involvement of the circadian system. In addition, a modulatory effect of melatonin on the circadian pacemaker might help in adjusting to changing photoperiod in species that migrate fast and/or along the east-west axis. Preliminary experiments with bramblings (Fringilla montifringilla) and garden warblers indicate that resynchronization times are shorter during the migratory season than at other times. In this paper we will review studies on melatonin and nocturnal migration focusing on our work on Sylvia warblers, and discuss how melatonin might mediate the different requirements of autumnal and spring migration.