Brood parasitism and competition for nest sites as factors in conspecific aggression in the Northern Flicker ( Colaptes auratus )

Several factors may explain patterns of conspecific aggression in breeding birds including mating systems, access to resources such as nest sites and risk of brood parasitism. In most species, the potential for social polygamy and/or extra-pair fertilizations leads to a common pattern in which intrasexual aggression is greater than intersexual aggression. I studied reactions to conspecifics in Northern Flickers (Colaptes auratus), a woodpecker with an unusual mating system of facultative polyandry and with no extra-pair fertilization. Experimental presentations of taxidermic models of both sexes to 26 male and 24 female flickers at their nesting cavity during the egg-laying stage showed that males physically attacked females with equal intensity as they did males. In females, intrasexual aggression was higher than intersexual aggression because females often assumed defensive behavior toward model males rather than attacking them. A high level of aggression to intruding females by both sexes is best explained by the risk of brood parasitism. Unlike the case in many species of birds, intrasexual aggression in male Northern Flickers is not linked to the loss of paternity or of a territory but seems to be driven by the risk of losing a nest cavity, a limited resource.