Investment Strategies of Breeders in Avian Cooperative Breeding Systems.

An individual's optimal investment in young depends partly on the number of individuals caring for the same brood. In cooperative breeders, the investment strategy of parents with helpers is variable. When parents maintain the same effort regardless of helper number, helper care is additive. When parents fully compensate for the care of helpers by decreasing their own effort, total care does not increase. A study of long-tailed tits Aegithalos caudatus showed that both parental strategies may occur within a species, depending on the number of helpers. A comparative analysis of 27 cooperative breeders was conducted to test the predictions of a graphical model that care is additive when nestling starvation is frequent and parents exhibit compensatory reductions in care when starvation is rare. Both predictions were supported. In this interspecific comparison, a species' mean group size was not associated with compensatory responses by parents. There was some evidence that males and females had different investment rules. Males tended to show compensatory reductions in care when adult survival rate was low. In contrast, while both sexes showed compensation when nestling starvation was infrequent, this association was significant only for females.