Patterns of natal dispersal, turnover and dispersal costs in eastern bluebirds

Abstract

Most hypotheses explain dispersal patterns in vertebrate populations based 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 also be modelled from the basic structure and demographic parameters of the resident population, independent of costs to dispersers. For example, 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 parents and availability of nest sites were subsequently examined. Females settled significantly further from the natal territory than did males and were more likely to emigrate from the local population. In addition, immigrant females outnumbered immigrant males within local populations. 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 excluded. Adult turnover rates did not affect dispersal of individual offspring or general philopatry rates within local populations (as a factor 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 adult survivorship.