Male rank, female breeding synchrony, and patterns of paternity in the boat-tailed grackle

Species in which males directly defend groups of breeding females often have extreme skew in observed male mating success. In only a few species, however, has a corresponding skew in fertilization success been confirmed. Furthermore, the ecological and social factors contributing to variation in fertilization success need investigation. This study examined competition for mates and paternity in the boat-tailed grackle (Quiscalus major). Observations at colonies of nesting females revealed that the top-ranking or alpha males performed more than 70% of the copulations. DNA fingerprinting indicated that alpha males sired less than 40% of nestlings. Nevertheless, analysis of band-sharing scores among nestlings from different nests suggested that alpha males sired more than three times as many offspring as any other individual male. Because few nestlings were sired by the nonalpha males that associated with colonies, females must have mated with other males while on trips away from colonies. Analysis of paternity within broods revealed that at least half of all females had their brood fertilized by more than one male. Alpha males' success at fertilizing eggs did not vary with the number of simultaneously receptive females within a colony. Our results suggest that male and female behavior in female-defense polygyny results from complex coevolution of the sexes.