Provisioning patterns in the cooperatively breeding acorn woodpecker: does feeding behaviour serve as a signal?

Abstract

Acorn woodpeckers, Melanerpes formicivorus, are cooperative breeders in which social groups consist of both nonbreeding helpers at the nest (offspring from prior reproductive attempts) and cobreeders of one or both sexes (usually siblings or a parent and his/her offspring). Regardless of composition, groups generally have one nest at a time at which all individuals participate in provisioning offspring. We tested the hypothesis that provisioning behaviour serves a signalling function used to gain social advantages within groups by enhancing dominance or social prestige, or by reducing the likelihood of being expelled from the group (‘pay-to-stay’). We found that birds adjusted their provisioning behaviour based on the activities of other group members by clumping their visits and by alternating their visits with other group members, thus synchronizing and coordinating provisioning within groups. Despite this evidence that acorn woodpeckers respond to the provisioning behaviour of other group members, analyses of feeding rates and patterns of overlap revealed no support for the hypothesis that provisioning functions as a signal to other group members in any of three ways: breeder males signalling to breeder females to increase their probability of mating; helpers signalling to other helpers to enhance their dominance or social prestige; or helpers signalling to breeders to reduce the probability that they will be considered ‘lazy’ and be evicted from the group. Our results add to previous studies that have thus far failed to support a signalling function for provisioning behaviour in avian cooperative breeders.