![]() Second, treatment with exogenous melatonin should reduce the intensity of Zugunruhe. Following this hypothesis, we made two predictions that could be tested empirically: First, if the birds were induced to interrupt Zugunruhe during migration, one should observe an increase of circulating melatonin at the same time. We hypothesized that melatonin is involved in the switch between the nocturnal activity typical for migration and the diurnal pattern of activity shown during all other stages of their annual cycle (such as breeding, moulting and wintering). In addition, the correlation between nocturnal activity and reduced melatonin levels was present in migratory blackcaps Sylvia atricapilla, but not in a non-migratory population of this species which showed some nocturnal activity. Pioneer studies of Eberhard Gwinner and his colleagues had shown that in Sylvia warblers the amplitude of nocturnal melatonin secretion is reduced when birds show Zugunruhe compared to other times of the year, (reviewed by ). In birds, melatonin and the pineal gland are major players in the regulations of circadian rhythms. Melatonin is a hormone secreted by the pineal gland (or epiphysis), and in most vertebrates is implicated in transducing the photic signal. Although some non-migratory species or populations may show nocturnal restlessness, only birds that migrate over hundreds or thousands of kilometers show the robust locomotor activity that may persist for weeks and that has been traditionally called Zugunruhe. In captive birds, the onset of migration is indicated by the periodic expression of intense restlessness at night. The adaptive aspects of migrating at night are debated, and may include a combination of factors including avoidance of overheating, thermal condition of the atmosphere, reduced predation risk, use of orientation cues at sunset or during the night, and optimization of foraging strategies. Nocturnal migration is common in many passerine species that are day-active during all other stages of their annual cycle. In birds breeding in the northern hemisphere, the importance of a timely arrival to the breeding sites could explain why the control of the migratory programme is more rigid in spring. The comparison with a similar study carried out with the same modalities during spring migration suggests that there are seasonal differences in the sensitivity of the migratory programme to hormonal factors. To our knowledge, this is one of the first demonstrations that the autumn migratory programme can be altered by hormonal manipulation in migrating birds. This study shows that the migratory programme of two Sylvia warblers can be manipulated by administration of exogenous melatonin and confirms that this hormone is involved in the control of migratory behaviour. The change in body mass during the time spent in the recording cage depended on food availability, but not on any of the other factors. Fuel deposits also had a strong effect on the amount of nocturnal activity: lean birds with a fat score of 1 showed significantly less Zugunruhe than fatter birds. ![]() ![]() The melatonin treatment significantly decreased the amount of Zugunruhe, while the availability of food only tended to increase the amount of Zugunruhe. ![]() Food availability in the cage (yes/no) was added as a second factor because previous work showed that it enhanced Zugunruhe. atricapilla subject to temporary captivity at an autumnal stopover site. Here we tested whether a non-invasive treatment with melatonin can alter Zugunruhe in wild garden warblers S. However, previous studies indicated that the response to melatonin manipulation could differ between spring and autumn migration, which are in fact separate life history stages. In a recent study we found no effects of melatonin administration on Zugunruhe during spring migration. Previous work with Sylvia warblers suggested an involvement of melatonin, a hormone associated with day-night cycles in most vertebrates. The mechanisms that control this major change in activity are mostly unknown. The switch in activity from purely diurnal to also nocturnal is evident even in caged birds that during migratory periods develop an intense nocturnal restlessness, termed Zugunruhe. A remarkable aspect of bird migration is its nocturnality, particularly common in Passeriformes. ![]()
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