Abstract: The ability of a single hormone to mediate suites of traits creates biological trade-offs within an organism with respect to hormone levels. The adaptive significance of hormone levels within a natural population can be ascertained through analysis of traits manifested by experimentally increasing hormone levels in an organism. If deleterious traits outweigh positive traits, then the hormone levels associated with those traits will not be selected. This study focuses on testosterone levels in the Dark-eyed Junco (Junco hyemalis) where it may be that deleterious effects of elevated testosterone on females are constraining the evolution of elevated hormone levels in the junco population. In particular, this study examined the effect of experimentally elevated testosterone (phenotypic engineering) on incubation, a trait with important fitness consequences and known hormonal components. We found that testosterone had no effect on incubation behavior and did not affect hatching success.