Natal dispersal is the movement of juvenile animals from their natal home range to the location of first breeding (Howard 1960), and this behavior plays a vital role in animal ecology, mating systems, and population dynamics. From an ultimate perspective, dispersal movements facilitate gene flow and reduce potential for deleterious effects of inbreeding. While these ultimate factors are reasonably well understood, the proximate mechanisms associated with juvenile dispersal are less clear. Thus, for my graduate research I hope to focus on proximate mechanisms of natal dispersal in birds. Using field experiments, I will evaluate if and how body condition and access to a reliable food source affect the timing of dispersal in young birds. In so doing, I will examine a critical, but as yet untested prediction of a model that attempts to explain dispersal in non-migratory species of birds, using western screech-owls (Megascops asio) as my focal species.
Western screech-owls are small, non-migratory birds that inhabit woodlands in both urban and rural landscapes where they produce broods that range in size from 3 – 6. These owls have been the subject of previous research to understand aspects of their post-fledging behavior. Following dispersal from natal territories, juveniles establish breeding territories, attract a mate, and start nesting in February/March of their first year of life. Dominance hierarchies established among nestlings within a nest ultimately translate into the pattern of dispersal, such that the most dominant individuals disperse before their subordinate siblings (Ellsworth and Belthoff 1999). If there is selection on owls to disperse as soon as capable to acquire a breeding territory before competitors, then dominant individuals likely gain an advantage from earlier dispersal. There is also evidence for endogenous factors that increase locomotor activity in screech-owls (Ritchsion et al., 1992, Belthoff and Dufty 1995, Ferriere et al., 2000) around the time of dispersal, and such increases are associated with increases in the glucocorticoid hormone corticosterone. These processes are thought to interact with body condition.
Based on these factors, Belthoff and Dufty (1998) developed a model of dispersal in western screech-owls that posits that corticosterone increases mediate the locomotor activity that underlies dispersal behavior. Juveniles in good body condition disperse when corticosterone increases, but birds in poorer condition will not; instead they increase foraging under the influence of the corticosterone (hyperphagia). Dispersal of their siblings in better body condition will reduce aggression and competition for food in the natal area, and this enables the remaining juveniles to achieve body condition needed for dispersal, and they subsequently depart natal territories later.
While there is correlative evidence for the model’s body condition prediction (i.e., dominant individuals disperse before subordinates), I want to test this aspect of the model experimentally.