Subadult Plumage Color of Female Tree Swallows (Tachycineta bicolor) Reduces Conspecific Aggression During the Breeding Season

Abstract

Abstract Delayed plumage maturation (DPM) in young males (in their second year of life: SY) of some species of birds has been found to reduce aggression directed at these younger males. Tree Swallows (Tachycineta bicolor) are unusual because it is SY females rather than males that display DPM; however, the functional significance for DPM remains equivocal. During the breeding season, we tested whether both male and female Tree Swallows behaved differently towards SY and older (after second year: ASY) female Tree Swallow models. When presented as a territory intruder, we predicted that ASY models would receive more aggression from resident pairs if ASY females are perceived as a greater competitive threat. When presented as a territory owner, we predicted that SY models would experience more territorial intrusions if SY plumage is perceived as a signal of inexperience in territory defense. When models were presented as intruders, resident females were more aggressive towards ASY models compared to SY model...