Parental Effort in a Mating System Involving Two Males and Two Females

The dunnock is a small passerine bird with a variable mating system. We focus on a system involving two females and two males. Each male has the opportunity to mate with each female. Each female has her own nest. We consider a general model in which the probability of a chick surviving to maturity increases at a diminishing rate with total parental effort invested in it, and the expected reproductive success of a parent from future broods falls off at an increasing rate with effort expended on the current brood(s). Given the number of young in each nest and the expected paternity of each male, we establish general results concerning the evolutionary stable patterns of male effort at each nest. We then compute these patterns for a specific parameterization of the model. Our results are in good agreement with the available data.