Lazy group members are substitute helpers in carrion crows

In many cooperatively breeding societies, helping effort varies greatly among group members, raising the question of why dominant individuals tolerate lazy subordinates. In groups of carrion crows Corvus corone corone, helpers at the nest increase breeders'' reproductive success, but chick provisioning is unevenly distributed among non-breeders, with a gradient that ranges from individuals that work as much as the breeders to others that completely refrain from visiting the nest. Here we show that lazy non-breeders represent an insurance workforce that fully compensates for a reduction in the provisioning effort of another group member, avoiding a decrease in reproductive success. When we temporarily impaired a carer, decreasing its nest attendance, the laziest non-breeders increased their provisioning rate and individuals that initially refrained from visiting the nest started helping. Breeders, in contrast, did not increase chick provisioning. This shows that lazy non-breeders can buffer a sudden unfavourable circumstance and suggests that group stability relies on the potential contribution of group members in addition to their current effort.