Interactions among females in polygynous yellow-headed blackbirds

Summary Two alternative hypotheses which attempt to predict how females within harems in polygynous species should behave toward one another were examined in a population of yellow-headed blackbirds (Xanthocephalus xanthocephalus). The hypothesis that females within a harem are co-operative was not supported. Females in large harems did not enjoy higher reproductive success than those in smaller harems and females within a territory did not initiate their nests close in time or space. The alternative hypothesis that females within a harem are competitive was also not supported. Nest density did not have a negative effect on female reproductive success, females within a territory did not initiate their nests far apart in time or space, and resident females were not very aggressive toward intruding female mounts during the settling period. The data support the idea that females settled independently of one another and had no effect on one another's reproductive success.