Dispersal costs set the scene for helping in an atypical avian cooperativebreeder

The ecological constraints hypothesis is suggested to explain the evolution of cooperative breeding in birds. This hypothesis predicts that the scene for cooperative breeding is set when ecological factors constrain offspring from dispersal. This prediction was tested in the atypical cooperative bre eding system of the long-tailed tit, Aegithalos caudatus, by comparing the degree of philopatry and cooperation in an isolated and a contiguous site w hilst experimentally controlling for confounding aspects of reproduction. N o difference was found between the two sites in the survival of offspring b ut a greater proportion were found to remain philopatric in the isolated si te. This difference was caused by greater philopatry of normally dispersive females suggesting, as predicted, that dispersal costs were greater from t his site. Furthermore, a greater proportion of males and females cooperated following breeding failure in the isolated site than in the contiguous sit e. Thus, as has been suggested for typical avian cooperative breeders, disp ersal costs, relative to philopatric benefits, appear to set the scene for cooperative breeding in long-tailed tits.