Does year-round territoriality rather than habitat saturation explain delayed natal dispersal and cooperative breeding in the carrion crow?

The ecological factors that promote delayed dispersal of offspring in cooperatively breeding bird species are poorly understood. While single population studies have supported the view that natal dispersal is delayed as a consequence of lack of suitable breeding vacancies (ecological constraints hypothesis), recent theoretical models claim that habitat saturation cannot be the main factor leading to kin sociality. The carrion crow (Corvus corone ssp.) is an ideal model to investigate the ecology of delayed dispersal. The occurrence of kin sociality is highly variable among European populations, and it is known to be determined by environmental rather than genetic factors. Here we compare juvenile dispersal, territory turnover, habitat saturation (number of competitors per breeding vacancy), variability of territory quality and territoriality of two crow populations that differ in social organization (extensive kin sociality in Spain vs. social monogamy in Italy). We found that philopatry occurred in the less competitive and less variable environment. A review of the information available in literature on other crow populations qualitatively supports this result. In crows, juvenile philopatry seemed to be associated with breeders’ year-round residency in territories. This association is widespread among birds and it has been previously explained through an ecological constraints perspective, with residency slowing territory turnover and therefore augmenting saturation. Our data do not support this explanation.

VITTORIO BAGLIONE, JOSE M. MARCOS et al., Journal of Animal Ecology (2005) 74, 842-851