Functional Importance of Plumage Badges as Intraspecific Signals in White-Crowned Sparrows (zonotrichia leucophrys oriantha)

Abstract

Status badges, such as bird plumage colors, are important parts of animal communication; they mediate intra- as well as intersexual interactions. Reliability of avian plumage badges is thought to be maintained by selective pressures, including social punishment. Costs, benefits, and resultant fitness tradeoffs are thought to maintain reliable status badges as evolutionarily stable signals. We tested this hypothesis during two breeding seasons (summers 2008-2009) in a population of Mountain White-crowned Sparrows (Zonotrichia leucophrys oriantha; MWCS) in Colorado (USA). Both sexes of this species possess a black and white striped crown that mediates interactions between juvenile and adult birds. Crown whiteness, expressed as the percentage of the crown that consists of white feathers, varies widely across individual sparrows. To test whether the reliability (the consistent transfer of information relating signal design and content) of crown whiteness in males is maintained by social punishment, we examined cost and benefits associated with experimentally manipulated phenotypes. We predicted status badge related tradeoffs to sender condition and social interactions. We conducted a series of territory intrusions / call playbacks using male sparrow decoys with manipulated crowns to measure social punishment costs. Resident MWCS males received two simulated territorial intrusions by the same mounted decoy (once with a ‘white-enhanced’ and once with a ‘white-reduced’ crown treatment; these were presented in random order on different days). Males responded with significantly increased aggression when presented with white-enhanced decoys. In a parallel experiment we tested the prediction that crown whiteness is (1.) associated with sender condition and variation in levels of corticosterone and (2.) under sexual selection. That is, we predicted that males with whiter crowns would gain reproductive benefits (measured as number of fledglings). In these experiments we experimentally enlarged or reduced the proportion of whiteness in males’ crown feathers. Crown manipulations had no significant effect on baseline, nor on post-stress series corticosterone levels. While there was a trend for more offspring in males with white-enhanced crowns, this relationship was not ii significant due to small sample sizes of recovered nests. Our data provide support for the social punishment hypothesis, yet show no significant relationship between stress response and crown characteristics. Social costs are therefore at least partially responsible for maintaining the reliability of crown whiteness as a status badge in male MWCS.