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 fingerprinted 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 broods 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.