The availability of male mates affects the social mating strategies of a predominantly socially monogamous passerine

We evaluated whether the availability of male mates affected mating strategies in a predominantly socially monogamous passerine, the grass wren Cistothorus platensis. We used the natural variation in adult sex ratio (ASR) and a male removal manipulation to assess if social polygyny was more frequent when male mates were less abundant. We evaluated the potential costs that females paid when breeding with a polygynous male by assessing how males distributed parental care between nests and analysing four correlates of breeding success (clutch size, nestling body condition, number of fledglings, and nest fate). Social monogamy was predominant when males were abundant while social polygyny was more frequent when the ASR was female-biased. The removal of males was associated with a high rate of social polygyny. Social polygyny occurred when males annexed the territory of a neighbouring female or when an unpaired female settled within the territory of a mated male. We identified several potential costs of social polygyny to females. Secondary females (females that laid eggs after the primary female) produced nestlings of lower body condition during the first half of the breeding season, received less help in feeding the brood, and increased their parental contribution. We suggest that while intrasexual competition may constrain social polygyny when males are abundant, reduced female life expectancy combined with moderate costs of male desertion may facilitate social polygyny when potential male mates are not available.