We tested one of the foundational hypotheses of the field of ecological immunology: that it is difficult for animals to simultaneously carry out two or more especially demanding physiological processes at optimal levels because of energy needs or other factors that cause tradeoffs among competing components of life history. We investigated possible effects of molt (a costly life-history stage that all birds share) on three physiological parameters in 32 male and female Brown-headed Cowbirds (Molothrus ater). We measured hematocrit, corticosterone levels (CORT), and the bactericidal competence (BC) of blood plasma by drawing blood from the birds before, twice during, and twice after their molt from late July to early September 2009. In accordance with our predictions, BC dropped during molt for both males and females. Interestingly, BC recovered faster after molt ended in males, and female BC remained depressed for the rest of our sampling period. Hematocrit also dropped during the molt but returned to initial levels after molt in both males and females. CORT dropped during molt, but the change was significant only for males. Our results highlight possible physiological consequences of molt in Brown-headed Cowbirds even when the birds are maintained in optimal conditions (i.e., shelter, ad libitum food and water, relatively low-stress environment). Furthermore, we suggest that the slower recovery of female immune function following molt may be related to higher female mortality resulting in the ubiquitous phenomenon of male-biased sex ratios in Brown-headed Cowbird populations.