Nest Predation Lowers the Polygyny Threshold: A New Compensation Model

A crucial objective in the study of territorial polygyny is to assess the Darwinian fitness of females mating with already-mated males relative to those selecting unpaired males. A lot of effort has been put into testing hypotheses that stress that females selecting already-mated males compensate for their lower number of fledged young by other fitness components against hypotheses proposing that there is no such compensation. Here, we have presented a model showing that the rate of nest losses, in the range normally found among passerines, might favor the fitness of secondary females more than that of monogamous females, thus decreasing the cost of selecting mated males. This relies on the assumption that males predominantly feed the young that hatch first, regardless of the female's status when she settled. The model finds support from a study of the great reed warbler carried out during 5 yr in Sweden. The validity of three assumptions underlying the model has been discussed.