Effects of social status on the dispersal behaviour of juvenile western screech-owls

Variation in timing and distance of dispersal movements of juvenile birds m ay result from differences in competitive ability. Dispersal by low-ranking juveniles may be initiated before dominants if the latter force subordinat e siblings from natal areas. Conversely, when vacant territories are limite d and are acquired on a first-come first-served basis, selection could oper ate on young to disperse as early as possible. In this case, dominant indiv iduals with priority of access to resources in the natal area will mature m ore quickly and are expected to disperse first. If costs of dispersal incre ase with dispersal distance, dominant juveniles are expected to disperse sh orter distances. Alternately, if there: are advantages to long-distance dis persal, then dominants, which are in better condition, should disperse furt her than subordinates. We examined effects of social rank on the timing and distance (to wintering area) of dispersal movements by juvenile western sc reech-owls, Otus kennicottii, in Southwestern Idaho. Based on observations of aggressive interactions made using video cameras attached to nestboxes, we assigned dominance ranks to nestlings within nine broods. We radiotracke d young throughout the postfledging period to determine order of dispersal, and we located them after leaving their natal areas to determine distances to apparent overwintering areas. In six of seven broods, for which dispers al information could be recorded, the most dominant juvenile dispersed firs t. Moreover, in five of seven broods, the least dominant individual was the last individual to disperse, and the order of dispersal matched the domina nce hierarchy in four of seven broods. In contrast, social status did not a ffect postfledging dispersal distance. We conclude that social dominance re lationships influenced the timing bf dispersal in juvenile western screech- owls but not distance travelled to overwinter sites. (C) 1999 The Associati on for the Study of Animal Behaviour.