Male‐biased dispersal, female philopatry, and routes to fitness in a social corvid

Abstract

1 Dispersal in many species is sex-biased. Greenwood (1980, Animal Behaviour, 28, 1140–1162) hypothesized that the direction of sex bias was dependent on the mating system, and suggested that female-biased dispersal in birds is associated with monogamy and resource defence by males, while mammalian male-biased dispersal is associated with polygyny and male defence of mates. 2 Cooperatively breeding brown jays Cyanocorax morio live in large territorial groups and are one of the few bird species known to exhibit male-biased dispersal. Survivorship of adults is high, and both males and females delay breeding for many years. 3 In this study, dispersers almost always moved to neighbouring social groups, rarely in response to a known breeding vacancy, and both sexes dispersed into groups with fewer members of the same sex than their origin group. 4 Dispersal and extra-group forays were strongly male biased. Forays were more common during nest building and egg laying, suggesting that males were monitoring mating opportunities, and many males later dispersed into the groups they visited. Genetic parentage analyses indicated that males mainly obtain paternity outside their natal groups through extra-group forays and dispersal. 5 The major routes to breeding status for females are inheritance of breeding positions on their natal territories and/or breeding there concurrently with an established breeder. Females usually dispersed on their own into groups where reproductive success had previously been poor. 6 Kin facilitation appears to affect male dispersal options: related males formed coalitions and tended to disperse to groups where male relatives were already established. 7 Male dispersal, female philopatry, and a matrilineal society correspond to three of Greenwood's seven predictions for a ‘mate-defence’ system, but overt mate defence itself is absent, territorial defence is communal, and genetic polyandry is common. Social behaviour appears to affect a reversal in sex-specific costs and benefits of dispersal, so that females benefit from philopatry and matrilineal inheritance, while males benefit from expanded dispersal options in part through kin facilitation, but without effective mate defence or clear cut sex-specific resource defence.