Proximate and ultimate mechanisms of red-winged blackbird (Agelaius phoeniceus) responses to interspecific brood parasitism

I studied behavioral mechanisms of an egg-acceptance and egg-ejection response to brood-parasitic eggs in a population of Red-winged Blackbirds (Agelaius phoeniceus) in western Nebraska. The host responses to real Brown-headed Cowbird (Molothrus ater) eggs and model Cowbird eggs were investigated on 1024 nests in marsh and riparian habitats during two field seasons. Seventy-three percent of the host females accepted and twenty-seven percent ejected the parasitic eggs. Distribution of both behavioral phenotypes was unequal in these habitats. Female responses to parasitic eggs were correlated with the number of eggs it had laid prior to the parasitic event. In the marsh habitat, females accepted the model eggs with a higher probability after they had laid their second or third egg than the first egg. This pattern was not observed for the females in the riparian habitat. The females\u27 responses to model Cowbird eggs were not correlated with the mean nest distance and mean area per nest in the marsh habitat. In the riparian habitat, mean area per nest and percentage of females that ejected the model eggs were positively correlated. My data suggest that the one-year old females (without assumed previous nesting experience) did not learn the egg-ejecting behavior from older females. My data indicate that females nesting in close proximity do not acquire the response to the parasitic egg from each other. Females of both age-classes that ejected the parasitic eggs spent more time close to their nests before they started to lay eggs. Females older than one year that ejected the parasitic eggs behaved more aggressively towards the parasitic eggs than did one-year-old females ejecting the model eggs