Effects on female reproductive success of familiarity and experience among male red-winged blackbirds

Evidence from a long-term study of red-winged blackbirds, Agelaius phoenicus, in Washington has indicated that females preferentially nest in territories of males that are familiar with their neighbours, perhaps because familiar males cooperate in nest defence. Data presented here from a long-term study in Ontario indicated that an average of 55% of territorial males returned from one year to the next, and of those, 81% reoccupied their former territories, so males were often familiar with their neighbours. Also 40% of breeding females returned to the study area between years (76% of those individuals returned to the same marsh), allowing the possibility of females recognizing males that were familiar with each other. However, no advantage to females, or preference by females, for familiarity among male neighbours was found. In fact, females were more successful on territories with one or more new neighbours because of reduced nest predation. These results, in conjunction with observations of male interactions with their neighbours during focal observations and model predator presentations, provided no evidence that familiarity among males was advantageous to females or facilitated cooperative nest defence by males. The eVect of familiarity among neighbours may vary geographically (in Washington and Ontario) because the principal species of nest predators diVer geographically and these predators may respond diVerently to variation in nesting density. Having new males as neighbours may be advantageous in Ontario because new neighbours are inexperienced and attract fewer females. In turn, local nest density is reduced, resulting in lower predation. When neighbours do help defend nests on their neighbour’s territory, they may be defending young they have sired through extra-pair copulations, so it may be prudent to continue regarding territorial neighbours as adversaries, even when their behaviour appears cooperative.