Limits to cooperative polyandry in birds

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Abstract
In communally breeding birds the number of helpers at the nest is less when helping depends on shared paternity (cooperative polyandry) rather than on collateral kinship with the brood (helpers are siblings, aunts or uncles). We suggest that this is because a helper’s relatedness to the brood decreases with an increasing number of helpers in the first case but not in the second. By using the dunnock, Prunella modularis, as a model system, we investigated why cooperative polyandry in this species rarely involves more than two males by removing females to increase male availability in the population. Females defended by just one male actively solicited matings from a second male who settled on their territory. By contrast, although widowed males also attempted to settle on territories already defended by two males, females usually refused to mate with them even though they had ample opportunity to do so. We show that a female would be unlikely to increase the total parental help she gained by sharing matings between more than two males because of the way males reduce their parental effort in relation to paternity loss, and suggest that mating with more males would also increase sexual harassment. We conclude that female choice may set a limit to cooperative polyandry in birds.