Contributions to care vary with age, sex, breeding status and group size in the cooperatively breeding apostlebird

Investment strategies in cooperative societies are often complex but should reflect individual variation in the costs and benefits of providing care. We examined the contributions to incubation and nestling provisioning by individual members of 16 groups of the cooperatively breeding apostlebird, Struthidea cinerea. Contributions varied in relation to the age, sex and breeding status of the carer, and with group size. Yearlings were less likely to help than older birds, and, when they did, spent less time incubating and fed the brood less often. Among adults (>= 2 years old), male helpers incubated less than breeders or female helpers. Mothers spent more time incubating than female helpers, and both incubated less when in larger groups. Mean incubation effort did not differ between fathers and male helpers, or between the sexes. Neither paternal nor male helper incubation effort was related to group size. There was no difference between adult categories in the likelihood of brood provisioning. However, male helpers provisioned at a higher rate than both female helpers and breeders. Overall, individual provisioning rates were lower in larger groups, although feeding rate per nestling increased, indicating partial adjustment to the increase in available help. Mothers and female helpers, and to some extent fathers, provisioned less when in larger groups. Because indirect benefits accruing to the sexes are expected to be similar, these observations suggest that brood provisioning may confer greater direct benefits to males than females. (c) 2006 The Association for the Study of Animal Behaviour Published by Elsevier Ltd.