Mate acquisition in the European blackbird and its implications for sexual strategies

Abstract

Female European blackbirds, Turdus merula have been shown to engage in mixed reproductive strategies and females do not seem to gain any material benefit from their extra-pair copulation behaviour. To test the hypothesis that such behaviour arises out of constraint on their choice of social mate, I investigate if and how these females are constrained in their choice at mate acquisition. I recorded patterns of territorial distribution and interactions between resident birds both before pairing and during the breeding season. Patterns of observed behaviour agreed with earlier descriptions of winter and breeding territoriality and provided clues to the underlying mechanism of mate acquisition. I conclude that the constraint on breeding success imposed by high nest predation promotes intrasexual competition for high quality nesting habitat between females prior to pair formation. Female distribution promotes intrasexual competition between males for territorial area overlapping females or for habitat likely to be occupied by females. Mate choice during pair formation is constrained by the outcome of this intrasexual competition and, typically, overlapping territorial males and females pair into social monogamy on a shared breeding territory. Although this ultimately leads to assortitive mating by competitive ability, low quality females paired to low quality males may improve upon the quality of their genetic mate by engaging in extra-pair copulations. These data suggest that the process of mate acquisition in urban European blackbirds constrains the choice of social mate and so generates sexual selection pressures to drive mixed reproductive strategies during the breeding season.