CORTICOSTERONE, BODY CONDITION AND LOCOMOTOR-ACTIVITY - A MODEL FOR DISPERSAL IN SCREECH-OWLS

A model that explains natal dispersal in resident screech-owls is pres ented and examined. The model is based on interactions among hormonal changes, body condition and social stimuli. It predicts that corticost erone, an adrenal glucocorticoid known to stimulate locomotor and fora ging activity, increases in blood plasma prior to dispersal through a combination of endogenous and exogenous events. This mediates the loco motor activity that underlies dispersal behaviour. Juveniles in good b ody condition (i.e. those with sufficient fat reserves) will disperse when corticosterone increases. Birds in poor body condition will not, but they will increase their foraging activity under the influence of corticosterone. Dispersal of siblings will reduce aggression and/or co mpetition for food, enabling the remaining juveniles to improve their body condition and disperse. Initial studies on screech-owls, Otus asi o and O. kennicottii, have produced results that are generally consist ent with the model. For example, captive juvenile screech-owls showed increased locomotor activity in the weeks leading up to the time when free-living juveniles are dispersing, and activity levels declined the reafter. Peaks in corticosterone corresponded with periods of high loc omotor activity (i.e. at the time of dispersal) in captive owls. Final ly, field studies indicate that dominant juveniles, which are presumab ly in better physical condition, initiated dispersal before their more subordinate siblings. (C) 1998 The Association for the Study of Anima l Behaviour.