Correlates and Fitness Consequences of Natal Dispersal in Swainson's Hawks

Natal dispersal is an important, yet often poorly studied, process in population ecology. Natal dispersal can affect population structure and individual fitness. We examined correlates of natal dispersal distances of a population of Swainson's Hawk (Buteo swainsoni) in northern California that has been monitored from 1979 to 2010. We examined the effects of sex, number of offspring fledged, population density, primary productivity around a nest site, and annual trends on dispersal from the natal site to the breeding site of individuals recruited into the breeding population. Female Swainson's Hawks dispersed farther than males. Natal dispersal was negatively correlated with primary productivity and was positively correlated with population density around the nest site. We found no correlation between natal dispersal distance and lifetime reproductive success of those individuals recruited into the breeding population, indicating no fitness costs of increased dispersal. We examined the potential genetic contributions to dispersal distance by examining differences in dispersal distance between full siblings. We found that the dispersal difference between siblings was significantly more similar than would be expected by chance. However, we detected no relationships between paternal or maternal natal dispersal distances and natal dispersal of offspring, although sample sizes were limited. We conclude that observed similarities in siblings' natal dispersal distance were likely a result of similarities in unmeasured environmental variables rather than of a genetic mechanism of natal dispersal. Perinatal conditions seemed to have significant effects on distances offspring dispersed, but increased natal dispersal had no apparent consequences on fitness.