On the Significance of Spring Migratory Restlessness in Caged Birds

Abstract

Caged migratory birds, normally migrating at night, develop nocturnal activity during the migratory seasons. In autumn, this migratory restlessness or often reflects the normal temporal course of actual migration in freeliving conspecifics. In spring, in contrast, persists throughout the normal breeding season and ends only with the beginning of postnuptial moult in practically all species studied. In the present paper, the interpretation of this so called summer Zugunruhe is discussed and three hypotheses are presented which might explain this phenomenon. These propose that nocturnal activity during the breeding season either (1) is not related to migration at all, but reflects a general tendency of the birds to perform various kinds of activities at night; or (2) is related to migration and results from the inability of caged birds to respond to stimuli which characterize their home area onto which they are normally imprinted in early youth; or (3) is related to migration and results from the inability of caged birds to perform reproductive activities. Although none of these three models can be excluded at present time, the available evidence favours hypothesis (3) as the most likely explanation of this phenomenon. Possible experiments for testing it are suggested.