Aggression of breeding eastern bluebirds (Sialia sialis) toward their mates and models of intra-and interspecific intruders

Hypotheses regarding the evolution of aggressive patterns during the breeding season in eastern bluebirds (Sialia sialis) were tested by placing models of eastern bluebirds and house sparrows (Passer domesticus) into territories of experimentally naive breeding pairs of eastern bluebirds during three stages of the nesting cycle. The responses of the resident males to models of male bluebirds decreased from the egg-laying to the nestling stage, and this response pattern can be interpreted as support for the protection-of-parentage hypothesis or the nest-site-defence hypothesis. The aggression of resident males to resident females during trials with models of male bluebirds weakly conforms to the pattern predicted by the protection-of-parentage hypothesis. The aggressive response to models of house sparrows also decreased from the egg-laying to the nestling phase in a manner consistent with the nest-site-defence explanation. No aggression toward mates by males or females was observed during trials with house sparrow models, an observation consistent with the predictions of the protection-of-parentage hypothesis.