Ecologically determined natal philopatry within a colony of great cormorants

Dispersal patterns of individuals within populations have implicationsfor the social and genetic structure of local populations.Knowing what factors determine individual dispersal behavioris essential for predicting how the population structure willbe influenced by environmental and demographic changes. Inthis study, I investigated whether the settling pattern of individuals breeding for the first time within a colony of greatcormorants 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 natalcolony showed strong philopatry toward their natal breedingsites. Because of the simultaneous strong fidelity of breederstoward their former breeding sites, this caused kin to clusterto some extent around the natal site. However, genetic factors(attraction to close kin) are less likely to explain natalphilopatry than ecological ones (attraction to the natal siteitself). Younger first-time breeders were more philopatric than older ones, in accordance with a decrease in the predictabilityof the quality of breeding sites with increasing time lags.Furthermore, males dispersed farther from the natal breedingsite than females. This result is contrary to what is generallyexpected for a breeding system where the male is dependent on a breeding territory for mate acquisition. I suggest thatthis sex difference could arise because first-time breedingmales are constrained from settling in the natal site by interferencecompetition with older males or because males are better informedabout alternative breeding sites of high quality within thecolony.