acase@tulane.edu / Understanding the causes and consequences of plumage ornamentation in birds has long been of interest to evolutionary ecologists. Although conspicuous plumage is often present year-round, and is therefore likely to be under selection throughout the annual cycle, it has been much better-studied during breeding than non-breeding seasons. Signaling function of ornaments may be distinct during the non-breeding season because of seasonal differences in ecological and evolutionary pressures. The red-backed fairywren is a small, insectivorous passerine bird from Australia in which males have variable plumage ornamentation: some males develop red/black plumage months before breeding, others do so immediately before breeding, and others retain cryptic brown plumage year-round. My thesis investigated the causes and consequences of this variation, which a focus on non-breeding season dynamics. In my first chapter, I characterized the hormonal basis - or lack thereof - for ornamented plumage for early-molting males, finding that androgen levels in both males and females regardless of molt status were similar during the non-breeding season. By expressing plumage ornamentation in the absence of elevated androgen levels, ornamented males may avoid immunological or other costs associated with androgens. In my second chapter, I found that early-molting males did not pay a cost in terms in terms of signal quality relative to males that developed plumage ornamentation later in the year, when resources might be more available. Instead, males that developed ornamented plumage months before breeding had putatively higher quality ornamentation during the breeding season, and I identified adventitious molt in these males as a novel mechanism for plumage modification. In my third chapter, I showed that ornamented males ranged over larger areas and had more social connections that unornamented males. These males also displayed to females months before the start of the breeding season, suggesting that space use during the non-breeding season may be in part driven by opportunities to interact with future reproductive partners. This work advances our understanding of the production mechanisms, signaling function, and behavioral correlates of plumage ornamentation during non-breeding periods. / 1 / Samantha Lantz