Skewed sex ratios in a newly established osprey population

With recent increases in the numbers of reintroduction projects, it has become crucial to know the main factors that allow colonization of new areas and prevent the extinction of small and reintroduced populations. Dispersal is one of the most important phenomena in population biology with consequences to the proportion of individuals that keep breeding in the natal population and the number of individuals that move to other populations to breed. We studied changes in offspring sex ratio and differences in dispersal patterns between the sexes in a reintroduced population of osprey (Pandion haliaetus). Results showed that at the beginning of the colonization process breeding pairs produced more males, which dispersed shorter distances and were more philopatric than females. However, with increasing breeding population size over the years, the offspring production was skewed to females, which dispersed longer distances from their natal area and tended to breed in different areas. Here we suggest that changes in offspring sex ratio during colonization processes, together with differences in dispersal pattern between sexes, could influence colonization rate and the probability of success of future reintroduction projects.