Proper functioning of the immune system is critical to an organism's fitness, but the immune system is expensive to develop and maintain. Organisms thus have to balance the costs of parasites against the costs of immunity. I examined the relationships among immune function, corticosterone (CORT), reproductive strategy, sexual signals and stress in an avian brood-parasite, the brown-headed cowbird (Molothrus ater: "cowbird"), and its close non-parasitic relative, the red-winged blackbird (Agelaius phoeniceus: "redwing") to understand how variation in immune function may reflect different life-history strategies.

In chapter 1, I investigated the relationship between reproductive strategy and immunity in cowbirds and redwings during the breeding and non-breeding seasons. Female cowbirds had significantly reduced immune function compared to male cowbirds and female redwings during the breeding season, whereas there were no differences between these groups in the non-breeding season. Female redwings, in contrast, had significantly higher bactericidal capacity (BC) and similar swelling response to phytohemagglutinin (PHA) compared to male redwings during the breeding season. In conjunction with previous work, these results suggest that female cowbirds trade off current condition as a result of their heavy investment in current reproduction.

In chapters 2 and 3, I examined the relationships between static signals and immunity in male cowbirds and redwings. Both species exhibited a significant negative relationship between the signal (song repertoire size in cowbirds and epaulet size in redwings) and BC. In cowbirds, CORT was positively correlated with song repertoire size, but was not related to BC, whereas in redwings, CORT was not related to epaulet size but was positively correlated with BC. BC is likely modulated by activity levels and social feedback based on signal expression.

In Chapter 4, I studied the effect of acute stress on the BC of cowbirds. Male cowbirds exhibit a significant decline in BC following handling stress and the administration of exogenous CORT without an associated stress event, whereas female cowbirds exhibited no change in BC following either treatment. Both CORT and corticosteroid binding globulins increased significantly following the non-stressful administration of CORT, and these increases were similar in males and females.