Life-history evolution is constrained by reproductive costs, as indicated by trade-offs between investing in current reproduction and future reproduction and survival. The physiological mechanisms driving these trade-offs, however, are not well understood. Oxidative stress is a candidate physiological mechanism for mediating costs that drive breeding season tradeoffs in migratory songbirds. Yet, few studies have focused on links between oxidative stress and reproductive effort. I hypothesized that reproductive effort generates oxidative stress (oxidative-cost hypothesis) and that oxidative stress shapes reproductive effort to mitigate the costs of reproduction (oxidative-constraints hypothesis). I examined whether variation in three oxidative measures (oxidative damage, antioxidant capacity, and oxidative stress) were associated with measures of breeding territory size and quality, parental effort, and reproductive success in male black-throated blue warblers at Hubbard Brook Experimental Forest, NH, USA. I found that oxidative stress functions as both a cost of and a constraint on reproduction. Across the breeding season, oxidative damage and oxidative stress decreased. Males with higher oxidative damage and oxidative stress had larger and higher quality territories, and males with higher antioxidant capacity delivered more prey biomass to nestlings. Males with higher-quality territories had lower oxidative stress and greater reproductive success. My results suggest that oxidative stress mediates the costs of reproduction in male black-throated blue warblers and, therefore, is an important driver of life-history tradeoffs in this migratory songbird. Delineating further relationships between oxidative stress and reproductive costs is critical to understanding life-history evolution in migratory songbirds.