Social Organization of Cooperatively Polyandrous White-Winged Trumpeters (Psophia leucoptera)

I observed a population of White-winged Trumpeters (Psophia leucoptera) in undisturbed rain forest in Peru for over 2,400 h between 1983 and 1987. At this site, I was able to habituate and band three groups of trumpeters and make occasional observations on four unhabituated groups. The habituated trumpeters lived in cooperatively polyandrous groups that defended large (x̄ = 72 ha) permanent territories against conspecifics. The habituated groups always contained a dominant male and female, and usually contained two unrelated subordinate adult males, one unrelated subordinate adult female, and the group's offspring. Only the dominant female contributed eggs to the clutch, and the group's adult males competed to obtain copulations with her. In the habituated groups, the dominant male obtained the majority of the successful copulations with the breeding female during her fertile period, and the beta male obtained a greater number of copulations than the gamma male. Individuals assisted to varying degrees with helping to rear the group's chicks. Subordinate males provided significantly more food to the chicks than the dominant male, and the subordinate female provided significantly less food than the dominant female, while the dominant male and female fed chicks equivalent amounts of food. Offspring usually helped raise one brood of siblings, but both males and females dispersed from their natal group at about two years of age, when they reached sexual maturity. The evolution of cooperative breeding in White-winged Trumpeters appears to be related to the need to defend large permanent territories to provide access to sufficient food during the dry season when resources are scarce. Defense of large territories results in a surplus of adults in the trumpeter population relative to the number of available breeding positions. The low probability of acquiring a breeding position for individuals that remain on their natal territory appears to result in sexually mature offspring dispersing from their natal groups and attempting to join new groups where they have some chance of breeding nonincestuously. The apparent need to have multiple adult males within a group for successful territory defense may explain why unrelated males are accepted into territorial groups, where they are able to copulate surreptitiously with the breeding female.