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 models. When presented as territory owners, ASY models received more aggression than SY models from intruding Tree Swallows, although we suspect that intruders were neighboring Tree Swallows with already established territories, as opposed to birds seeking to settle within the territories of our models. Overall, our results suggest that DPM in female Tree Swallows is an adaptive trait that potentially reduces intrasexual competition for SY females during the breeding season.

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