Reproductive success increases with group size in cooperative carrion crows, Corvus corone corone

The effect of group size and the number of helpers on reproductive success is crucial to understand the evolution and maintenance of cooperative breeding. In northern Spain, carrion crows form kin-groups (three to nine individuals) where up to five individuals contribute to rearing the young. Using data from 99 different territories, followed over 10 years, we showed that group size was positively correlated with the annual number of fledglings produced, after controlling for the potential confounding effect of territory quality. This occurred through: (1) an increased probability for larger groups of renesting after early nest failure; (2) a higher probability of nest success; and (3) a higher number of fledglings produced in successful attempts. Video-recorded observations at the nests showed that chicks received more food in larger groups during the first 10 days of life, when the risk of starvation is highest, suggesting a role of allofeeding by additional carers in augmenting the reproductive success of a group. In crows, indirect and direct benefits through increased production of young are therefore available to nonbreeding group members.