Dispersal restlessness: evidence for innate dispersal by juvenile eastern screech-owls?

Proximate factors responsible for the initiation of natal dispersal are poorly understood. Parental aggression, one possible factor, does not appear to initiate natal dispersal in young eastern screech-owls, Otus asio. Instead, intrinsic or innate factors may influence dispersal, resulting in increased activity near the time of dispersal. To examine the role of endogenous factors, screech-owl nestlings (approximately 15 days of age) were isolated and their activity levels monitored with digital pedometers for 20 weeks. Activity levels of captive owls (N=5) typically increased prior to the time they would have dispersed if not taken into captivity, then decreased following this time. Similarly, activity levels of free-living, radio-tagged juveniles (N=3) increased until shortly after dispersal, then decreased during the postdispersal period. Captive owls gained mass for several weeks following their estimated fledging date, and then, despite ad libitum feeding, either sustained or lost mass around the time when dispersal normally occurs in free-living birds. Captive owls again gained mass during the post-dispersal period. Reduced mass near the time of dispersal may be adaptive in that lighter birds may be more competent in flight and, therefore, more capable of dispersing. The effects of extrinsic factors other than parental aggression are currently unknown, but these results suggest that natal dispersal in eastern screech-owls is influenced by intrinsic factors and appear to support a 'dispersal restlessness' model for the initiation of natal dispersal.