PATTERNS OF NATAL DISPERSAL, TURNOVER AND DISPERSAL COSTS IN EASTERN BLUEBIRDS

Most hypotheses explain dispersal patterns in vertebrate populations b ased upon an assumption that individuals incur costs when moving away from their natal territories. The geometric distribution of dispersal distances that typically reflects observed dispersal, however, may als o be modelled from the basic structure and demographic parameters of t he resident population, independent of costs to dispersers. For exampl e, sex-biased natal dispersal might then be explained by differential turnover rates of adult males and females. Patterns of natal dispersal were determined for a South Carolina population of eastern bluebirds, Sialia sialis, from 1985 to 1991 and relationships between different measures of dispersal and initial reproductive success, survival of pa rents and availability of nest sites were subsequently examined. Femal es settled significantly further from the natal territory than did mal es and were more likely to emigrate from the local population. In addi tion, immigrant females outnumbered immigrant males within local popul ations. The initial reproductive success of philopatric and dispersing second-year males and females did not differ, but the possibility of high dispersal costs associated with mortality alone could not be excl uded. Adult turnover rates did not affect dispersal of individual offs pring or general philopatry rates within local populations (as a facto r determining overall territory availability). Therefore, alternative factors, such as competitive interactions or intrinsic mechanisms, may be more likely proximate causes of natal dispersal patterns than adul t survivorship. (C) 1996 The Association for the Study of Animal Behav iour