The optimal degree of parental care asymmetry among communal breeders

Intraspecific brood parasitism and cooperative breeding can be considered extremes on a continuum of parental care investment by a secondary female that visits the nest of a primary female. However, we lack a theoretical framework that predicts exactly where a secondary female will fall along this continuum. In this paper I present a model that predicts the fraction of a nesting period during which a secondary female is expected to stay and provide parental care. To develop this model, I use the basic outline of a patch model describing optimal residence time. Nests are treated as patches and the currency is offspring rather than energy. Females are expected to allocate parental care investment in such a way that the rate of fitness gain is maximized over the course of a breeding season. I begin with a one-patch type model that predicts the duration of parental care investment by primary females in their solitary nests. I use a similar approach to predict the duration of parental care by secondary females that visit the nests of primary females. One general result is that residence time increases as the rate of nest encounter decreases. Residence time decreases as reproductive skew increases, as relatedness decreases, and as fitness payoffs become less delayed. Finally, I develop a two-patch type model, where secondary females have the option of creating their own nests and must allocate parental investment between a host nest and their own. I derive a general expression for the optimal residence time of secondary females at a host nest, reflecting the degree of asymmetry in parental care between a pair of communally breeding females.