Reproductive success and survivorship of the Galapagos Hawk Buteo galapagoensis: potential costs and benefits of cooperative polyandry

Abstract

The Galapagos Hawk Buteo galapagoensis is one of few species showing cooperative polyandry. As many as four males may mate with one female and share a territory and nest, Such groups, on average, produce more young than monogamous pairs, but on a per male basis (assuming that each male of a group has an equal chance to fertilize each young) polyandrous males produce fewer young per year than monogamous males. Territorial breeding birds show higher yearly survivorship (90%) than non‐territorial, non‐breeding birds (50% or less). All territorial birds seem to remain on their territories for life, and monogamous pairs actually constitute the remnants of polyandrous groups. Some potential factors producing such an unusual system are discussed. Limited breeding space and high mortality among non‐territorial birds seem to work in favour of group breeding in this and other cooperative species. Polyandry may be favoured because of the reduced value of extra females in hawk breeding behaviour. Equal‐status males may occur because of the lack of kinship among cooperating males and the high potential genetic cost of being a helper in a long‐lived, low fecundity species. Other possible factors favouring group breeding are territory acquisition and the variable nature of the Galapagos climate.