Iridescence is a dynamic trait where color changes depending on the viewing angle. In birds, it is produced by the structure of the keratin, air and melanin within the feather barbule. Feather iridescence is widely distributed among modern birds. It evolved multiple times and is currently found in 1974 species (about 20.3% of birds) globally. Testing how iridescent plumage correlates with ecological and physiological patterns can help to elucidate the drivers that have shaped its current distribution and identify support for potential functions of iridescent feathers. In Chapter 1, we leverage phylogenetic methods to test hypotheses about the non-signaling functions of iridescent plumage. We found iridescence is significantly associated with aquatic lifestyle. Iridescent birds are overrepresented among species that spend much of their time on and foraging within water. Additionally, in species where both males and females are iridescent, we found that birds are more likely to have iridescent plumage in areas of higher solar radiation. Therefore, iridescence may play a role in solar protection. In the subsequent chapters, we explore the covariates of within-population variation of iridescent plumage in female free-living tree swallows. In Chapter 2, we evaluate how color metrics of hue, saturation and brightness along with an understudied quantitative measure of iridescence – directionality – predict adult phenotype and reproductive performance. We determine that bluer females initiate clutches earlier, and that more saturated females hatch larger broods and fledge more offspring. However, the directionality of dorsal plumage was not predictive of phenotype or performance. These results highlight that the iridescent dorsal plumage may act as a social signal in tree swallows, but directionality does not signal adult phenotype or reproductive performance. In Chapter 3, we examine whether iridescent plumage covaries with the hormonal stress response. We find that females with more saturated dorsal plumage mount a stronger acute stress response, and that females exposed to a short-term elevation in the hormonal mediators of the stress response (corticosterone) grow greener feathers in the subsequent year – although the directionality of their feathers was unaffected. These results suggest that plumage may provide information about the stress response in female tree swallows.