Classic explanations for polygyny consider habitat, genetic makeup and paternal care, but little attention has been paid to social inertia. We studied facultative social polygyny in a population of Blue Tits Cyanistes caeruleus with a low rate of polygyny (3% of males across 12 years). Occurrence of polygyny was best predicted by social turnover after the disappearance of one or more individuals; habitat quality and individual phenotypic traits were unimportant. Females settled as secondary females with a male and in an area they had previously associated with, indicating a role of familiarity in the formation of polygyny. Females mated to polygynous males received less help feeding nestlings. Reduced paternal care is potentially costly, because these females survived less well than females mated to monogamous males. Reduction in care occurred at both primary and secondary nests although polygynous males rarely divided their care between nests. On any given day during the nestling period, polygynous males were more likely than monogamous males to not feed nestlings at all and – as a consequence – polygynous males provided less care in total than monogamous males. Polygynous males rarely intermingled visits at both nests, suggesting a cost of switching between feeding locations. Both primary and secondary nests had reduced fledging success, even after controlling for variation in habitat quality and paternal care, due to a higher rate of nest failure. Paternity loss was higher at secondary nests, presumably because the secondary female copulated with a previous mate that disappeared. Due to increased paternity loss and reduced fledging success polygynous males did not sire more fledglings than monogamous males in a given season. Thus, the benefit of social polygyny for males seems limited. Social polygyny in our population probably arises as a by-product of widowed females settling as secondary mates with a familiar male or in a familiar area, thereby making the best of a bad situation.
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Vol. 109 • No. 1