‘Elepaio subadult plumages reduce aggression through graded status-signaling, not mimicry

Abstract

The 'Elepaio (Chasiempis sandwichensis) is a monarch flycatcher endemic to the Hawaiian Islands. Both sexes have a two-year delay in plumage maturation, with distinct first-year and second-year subadult plumages. This study tested hypotheses for the evolution of delayed plumage maturation in 'Elepaio by presenting adult males with models representing different ages and sexes in the breeding and nonbreeding seasons. The subadult plumages of 'Elepaio appeared to serve as honest, graded signals of status that reduced aggression from dominant adults. Both first-year and second-year subadult male models were attacked less than adult male models, and aggression increased linearly with age of male models. Neither subadult plumage appeared to function in sexual mimicry or juvenile mimicry. Second-year male models were attacked more than adult female models; neither subadult plumage resembles adult plumage of the opposite sex. First-year male models, which are juvenile-like, were not attacked more than adult female models, but juvenile mimicry cannot account for the behavior toward second-year male models and is a less parsimonious explanation. Aggression toward models was much higher in the breeding season, and all models were treated similarly in the nonbreeding season, suggesting competition for mates has been the primary selective force shaping the evolution of the 'Elepaio's plumage coloration. Models made from color pho- tocopies can be a useful alternative to museum specimens in behavioral studies of rare species or those in which damage to specimens during model presentation would occur.