Welklin, Joseph Frederick2021-03-152021-03-152020-12Welklin_cornellgrad_0058F_12384http://dissertations.umi.com/cornellgrad:12384https://hdl.handle.net/1813/103273360 pagesSexual selection theory proposes that ornaments used in mate choice should convey honest information about the quality of a signaler to conspecifics. In my dissertation, I address two major hypotheses for how signal honesty is maintained, the social cost hypothesis and the physiological cost hypothesis, using the Red-backed Fairywren (Malurus melanocephalus). Male Red-backed Fairywrens typically exhibit ornamented red-black breeding plumage, but this species offers unique opportunities to test the mechanisms that maintain the honesty of ornamented plumage because three-quarters of all one-year-old Red-backed Fairywrens spend their first breeding season in unornamented, brown plumage similar to that of females. Ornamented males obtain higher reproductive success than brown males, presenting the question: why do some males delay the acquisition of ornamented plumage when there are clear benefits to ornamentation? Here I address how social and physiological costs can help explain variation in ornamentation and reproductive success we observe across males.To test these hypotheses, I first had to understand the social and environmental dynamics of the non-breeding season, the period when males acquire ornamented plumage. My first chapter quantifies the non-breeding social environment and its relationship to climate variation, finding Red-backed Fairywrens form family groups during the non-breeding season, that these family groups interact in larger social communities, and that the rate of interactions among these groups is influenced by climate, especially rainfall. In my second chapter, I test whether environmental cues or social cues are associated with the timing and the production of ornamented plumage, finding that environmental cues are important in determining when older males obtain ornamented plumage within a year, but that only social cues, particularly pairing status, were important for determining whether or not a one-year-old male acquired ornamented plumage. Finally, in my third chapter I test the social and reproductive consequences of ornamented plumage expression using an experimental approach, finding that the production of ornamented plumage is socially costly in Red-backed Fairywrens. Combined, these results provide further insight into how the honesty of mating signals can be maintained and the forces that lead to variation in signal production within populations of the same species.enClimateFairywrenHonest signalingNon-breeding seasonSexual selectionSocial environmentdissertation or thesishttps://doi.org/10.7298/hhvs-s237