Background/Question/Methods Life history theory assumes that reproduction is expensive and competes for resources with other costly activities such as immune defense. Human-induced environmental variability may exacerbate this trade-off when resources become limiting. Evidence from laboratory and field studies show that habitat degradation increases stress levels, parasite loads, and malnutrition of animals. This study conducted during the summers of 2003 and 2004 examined the mechanisms through which food availability, stress, and parasite load interact to impact individuals within a wild population of White-crowned Sparrows (Zonotrichia leucophrys oriantha) during the breeding season. I experimentally reduced host parasite loads and manipulated food availability to study the effects on circulating corticosterone, immune function, and blood parasite loads. A standard stressor was applied to each bird to measure mean baseline circulating corticosterone, post-stressor corticosterone, and change in corticosterone to assess an individual's sensitivity to a stressor. Birds were injected with a non-pathogenic foreign protein, phytohemagglutinin (PHA), in the wing web and swelling was measured to assess strength of the cell-mediated immune response. I collected blood samples to quantify blood parasite intensities. Results/Conclusions Males were more sensitive to the applied stressor during the early breeding season than late breeding season, and immune responsiveness increased with time of season. Infection status with blood parasites significantly increased baseline corticosterone and decreased wing web swelling. Additionally, drug treatment significantly lowered mean baseline corticosterone in males only and increased wing web swellings in both sexes. Males were more sensitive than females to the applied stressor overall, and there is no significant difference in mean wing web swelling between the sexes. Birds who received both drug treatment and food supplements throughout the season had significantly higher wing web swellings than birds who did not receive either treatment. Environmental variability in the beginning of the breeding season could potentially affect immune responsiveness, which in turn influences a bird's susceptibility to relapse with a chronic infection or becoming acutely infected with blood parasites. Once infected, birds with blood parasites then experience significantly higher baseline stress levels than uninfected birds. Furthermore, it appears as if males and females are modulating their stress responses distinctly potentially due to the different reproductive costs the sexes experience throughout the breeding season.