Naturally occurring aggression between female eastern bluebirds (Sialia sialis) is dramatic, resulting in severe injuries and even death. Furthermore, aggression among bluebirds is usually sex specific: males attack males, females attack females. We hypothesized that the primary function of female-female aggression is to guard against the threat of intraspecific egg dumping and that, in this context, same sex aggression is related to the possibility of advantages for males of parasitism (egg dumping) of their nests. Our hypotheses to explain variation in naturally occurring aggression predict temporal variation in aggressive tendency within nest cycles and between the sexes depending on asymmetries in threats to the residents. We report the results of experimental trials in the field designed to determine temporal variation in the aggressive tendencies of resident females to models of intruder females of two species, eastern bluebirds and brown-headed cowbirds (Molothrus ater). Both species dump eggs in the nests of bluebirds. Female aggression to eastern bluebird models is greatest during early stages of nest cycles; the patterns are most consistent with protection against egg dumping and protection of nest sites from usurpation. Male residents seldom attack female eastern bluebird models, but often attack models of female brown-headed cowbirds, a result inconsistent with the hypothesis that patterns of differential parental care control aggression of female and male residents.