Dominance Hierarchies and Helper Contributions in Harris' Hawks

Abstract

We examined dominance hierarchies, mating relationships, and helping by individually marked Harris' Hawks (Parabuteo unicinctus) at 64 nests in Arizona (1984-1988). Dominance hierarchies were linear and always contained an adult female as the most dominant member. We placed hawks into three dominance categories (alpha, beta, and gamma) based on frequencies of supplanting. Pairs without helpers (16%, n = 64) and groups that contained a beta male helper (63%) were the most common social structures. Some groups (11%) contained a second alpha female (alpha-2) that was subordinate to the alpha female and dominant over the alpha male. Of 68 observations of copulatory behavior, 45 (66%) were copulations between alpha males and alpha females. Beta males attempted to copulate with alpha females (23 observations; 34%) but were unsuccessful because of a lack of female compliance. Observations of movements by marked hawks suggested that most gamma helpers were offspring of the alpha pair and that beta males and alpha-2 females were probably unrelated to the alpha pair. Alpha hawks restricted access to the nest by both beta and gamma helpers, but beta males were tolerated closer to the nest than were gamma helpers. Consequently, beta and gamma helpers rarely participated in the direct care of eggs and young but were active in procuring prey, transporting prey to the nest area, and defending the nest from predation by Great Horned Owls (Bubo virginianus).