Effects of food-deprivation on migratory restlessness and diurnal activity in the garden warbler Sylvia borin.

The present study describes the effects of food-deprivation on migratory restlessness and diurnal activity in the garden warbler, a long-distance migrating passerine. Fourteen first-year birds were held from August through March in individual registration cages and exposed to daylength changes experienced by birds that winter at the equator. With the beginning of the fall migratory season the birds increased their body weights due to fat deposition and showed nocturnal migratory restlessness characteristic for this species. After the birds had deposited more than about 4 g of fat they were food-deprived until their body mass had declined to a value about 1 g above fat free mass. Subsequently they were re-fed. This procedure was repeated through fall and winter, beyond the termination of spontaneous fall migratory restlessness. - Food deprivation consistently caused an increase in duration and intensity of nocturnal activity. After the end of spontaneous migratory restlessness and before the onset of winter molt it led to a reinduction of nocturnal activity. When nocturnal activity increased, the duration of diurnal activity decreased, whereas its intensity increased. With regard to the performance of free-living migrants these results suggest (1) that nocturnal migratory activity is enhanced in birds travelling across areas with unprofitable resting places; (2) that nocturnal migratory activity may be reactivated in birds that have settled for the winter in an area with deteriorating food supply; and (3) that the pattern of diurnal activity may change conspicuously when nocturnal migratory activity is enhanced or resumed.