Temporary caging results in reduced levels of circulating melatonin in migratory robins

The hormone melatonin, a main component of the avian circadian system, plays an important role in the physiological transitions that accompany activation of the migratory phenotype in passerine birds. Most small passerines migrate at night when circulating concentrations of melatonin are elevated. Previous work measured nocturnal melatonin levels of migratory birds only in captive animals, because free-living individuals are usually caught in the daytime. In this study, we compared nocturnal melatonin levels of European robins (Erithacus rubecula) caught during the day and held in cages overnight with those of birds that were caught at night and sampled immediately. We found that circulating melatonin at night was lower in birds held in cages compared with birds that were actively migrating. This result suggests that temporary caging affects the melatonin system and that, in nature, melatonin levels could be generally higher than those previously described by studies on captive birds.