Reciprocity in the green woodhoopoe ( Phoeniculus purpureus)

Green woodhoopoes ( Phoeniculus purpureus) exhibit behaviours that appear to meet all the criteria of reciprocity. In this species social units typically are composed of one breeding pair plus a variable number of helpers. A helper (X) that feeds nestling Y often is repaid by Y via Y's critical assistance in the acquisition of territorial ownership and breeding status by X; Y later also feeds the offspring of X. This pattern follows whether or not X and Y are related. Unrelated adult woodhoopoes of the same sex also may merge to form new social units, with the younger, lower ranking bird helping the older to acquire and retain territorial space. Later, it also provides care for the older bird's offspring. It is here suggested that a population structure based on a combination of cooperatively-breeding groups and high rates of mortality have favoured the evolution of reciprocal exchanges between unrelated individuals.