Ecologically determined natal philopatry within a colony of great cormorants

Dispersal patterns of individuals within populations have implications for the social and genetic structure of local populations. Knowing what factors determine individual dispersal behavior is essential for predicting how the population structure will be influenced by environmental and demographic changes. In this study, I investigated whether the settling pattern of individuals breeding for the first time a colony of great cormorants was determined by ecological or genetic factors. Furthermore, I examined the possible effects of age and gender. First-time breeders that came back to breed within their natal colony showed strong philopatry toward their natal breeding sites. Because of the simultaneous strong fidelity of breeders toward their former breeding sites, this caused kin to cluster to some extent around the natal site. However, genetic factors (attraction to close kin) are less likely to explain natal philopatry than ecological ones (attraction to the natal site itself). Younger first-time breeders were more philopatric than older ones, in accordance with a decrease in the predictability of the quality of breeding sites with increasing time lags. Furthermore, males dispersed farther from the natal breeding site than females. This result is contrary to what is generally expected for a breeding system where the male is dependent on a breeding territory for mate acquisition I suggest that this sex difference could arise because first-time breeding males are constrained from settling in the natal site by interference competition with older males or because males are better informed about alternative breeding sites of high quality within the colony.