Intersexual competition in a polygynous mating system
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In the facultatively polygynous European starling (Sturnus vulgaris) males attract from one to four mates. Males gain by mating polygynously because they produce more offspring by doing so. This is true also genetically, since polygynous males on average father the same proportion of offspring as monogamous males. However, the marginal benefit of attracting additional mates is negatively affected by extra-pair parentage, which is slightly higher in males' secondary broods. Whereas mating as a secondary female is a better option for floater females than either not reproducing at all or reproducing as a brood parasite, already mated females suffer a cost when their mates attract additional mates. This is because they have to share the parental care provided by the male. Males divide parental investment primarily in relation to the timing of the broods produced by their females, investing mainly in the earliest brood. Male parentage has little effect on male care. Already mated females use aggression to prevent floater females to establish a pair-bond with their mates. The mating system in the starling results from the differing interests of males and females. To be able to construct models predicting when a particular mating system should be expected, more has to be learnt about the costs of male mate attraction and nest defence behaviour and female aggression.