Each year, hundreds of thousands of shorebirds use Delaware Bay, on the northeast coast of the United States, as a final stopover before migration to breeding areas. The bay provides them with abundant Horseshoe Crab (Limulus polyphemus) eggs, which they use to gain the fat stores necessary for continued migration and subsequent survival on the breeding grounds. However, abundant food attracts dense mixed-species flocks, which may facilitate pathogen transmission, and migration itself may suppress immune defense. Despite the potential importance of disease risk and immune function during migration, little is known about how immune function changes during stopover in migratory shorebirds. To examine this, we measured constitutive immune function in Red Knots (Calidris canutus rufa) during stopover in Delaware Bay. We found lower total leukocyte, lymphocyte, and monocyte concentrations, complement-mediated lysis, and haptoglobin activity in birds recovering protein after migration than in birds storing fat to fuel subsequent flight. We discuss two possible reasons for this result. First, fueling birds may have an increased rate of infection or may be bolstering immune defense in response to high antigen exposure. Second, recovering birds may be immuno-compromised because of the physical strain of migratory flight or as a result of adaptive tradeoffs between immune function and migration.