Cooperation and competition: nepotistic tolerance and intrasexual aggression in western bluebird winter groups

Two hypothesized benefits of delayed dispersal are access to resources and prolonged brood care (or??parental nepotism). Resource abundance (mistletoe wealth) is a key factor influencing whether sons stay home in western bluebirds, Sialia mexicana, but nepotism is also observed. Western bluebird sons commonly remain in their family groups throughout the winter, whereas daughters usually disperse before winter. Because pairing often takes place in winter groups, with newly formed pairs settling on exclusive all-purpose territories in spring, selection for sexual competition and nepotism co-occur and may simultaneously influence patterns of aggression within groups. We measured aggression at mealworm feeder stations, finding evidence of (1) intrasexual aggression against unrelated group members by experienced breeders of both sexes and (2) nepotism towards sons and daughters by experienced breeder females but not by experienced breeder males. Females showed much higher levels of aggression towards same-sex immigrants than males did. Experienced breeder males did not evict their sons from the natal territory, but they were 12 times more aggressive towards sons than breeder females were towards daughters. They were also equally aggressive towards sons and immigrant males, suggesting that local breeding competition and the benefits of intrasexual dominance counter the benefits of paternal nepotism towards sons. ?? 2009 The Association for the Study of Animal Behaviour.