| The existence of elaborate plumages in birds has traditionally been explained as a result of mate choice by females for these traits in males. Ornate plumage traits are thought to be costly to produce and maintain, allowing females to gain accurate information about potential mates based on the expression of these traits. Female plumage traits, on the other hand, are less understood and have been rarely studied as having signaling potential. In an attempt to reduce this gap in knowledge, I investigated the potential signaling function of specific plumage characteristics in female tree swallows (Tachycineta bicolor). Tree swallows are one of only two species of North American passerines where females, rather than males, have delayed plumage maturation (DPM). Females in their second year of life (SY) display mainly brown "subadult-coloured" feathers on their dorsal surface, while older (after-second year of life: ASY) females primarily have iridescent blue-green "adult-coloured" feathers. There is, however, much variation in the number of adult-coloured feathers in the dorsal plumage of SY females. To date, only one study has experimentally examined the potential signaling function of DPM in female tree swallows, and no study has yet examined variation in the amount of DPM exhibited among SY females. Although plumage colour of ASY female tree swallows is less variable compared to SY females, such variation in ASY females is related to reproductive success, making both SY and ASY female plumage characteristics potentially indicative of quality. In this thesis, I investigated 1) if conspecifics behave differently towards SY and ASY females based solely on plumage colouration, indicating that DPM could signal subordinate status and inexperience during the breeding season, 2) if variability in the amount of DPM in SY female tree swallows is a condition-dependent trait and therefore possibly a target of sexual selection, and 3) whether variation in plumage traits of ASY female tree swallows is related to quality and thus could indicate how females choose to balance investment between life-history traits in this species. I found that the subadult-coloured plumage of SY female tree swallows elicits significantly less aggression from resident female tree swallows during the nest-building stage of reproduction compared to the adult-coloured plumage of ASY females. These results suggest that the plumage of SY females is an adaptive trait that signals subordinate status to conspecific females, potentially reducing intrasexual competition for SY females during the breeding season, increasing their chances of successfully breeding. Separately, when examining variation in the amount of DPM exhibited by SY females, I found that SY females with more adult-coloured plumage were in poorer body condition, had lower reproductive success and were mated to less UV-blue coloured males. I propose that increased agonistic encounters may account for the reduced reproductive success and condition of more adult-coloured SY females in my study. When examining the condition-dependence of ASY female plumage characteristics, I found that ASY females with more UV-blue colouration had lower reproductive success, while those with more UV-blue and brighter plumage had more parasites in their nests. These results, although unexpected, may still agree with current knowledge of the signaling function of ornamentation in birds if ASY females are experiencing costs associated with higher social status or sexual selection. Overall, my findings demonstrate that the adult-coloured plumage of female tree swallows is a potentially costly trait that elicits increased intrasexual aggression. As SY females with less adult-coloured plumage and ASY females with less ornamented adult-coloured plumage had higher reproductive success, my findings demonstrate an adaptive function of less ornamentation that has rarely been reported, increasing the current knowledge on the communicative function of plumage ornamentation. |
