Already mated females constrain male mating success in the European starling

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Abstract
Most models explaining polygyny in birds have concentrated on variation in male or territorial quality, ignoring the role of females in maintaining monogramy. Although field observations have suggested that already mated females may maintain monogamy by either behaving aggressively toward prospecting females or by interupting male mate attraction behaviour, no experiments have been done to test if already mated females constrain the mating success of their mates. In this study of the European starling (Sturnus vulgaris), the possibility for already settled females to defend their mating status was manipulated by changing the distance between their mates' potential breeding sites. Solitary breeding males were given an extra nest site at different distances from their present one: less than 2 m, 2-5.5 m and 7.5-15 m. The distance between nest sites was the most important determinant of male mating success; few males became polygynous when nest sites were close together whereas most became polygynous when nest sites were further apart. In addition, secondary females were able to lay earlier in relation to the primary females when their nest sites were further away from the primary females' nests. These results support the hypothesis that already mated females constrain the mating success of their mates.