Because climatic patterns in temperate regions are generally predictable, species can allocate resources adaptively among competing physiological processes before environmental conditions change. In the semi-arid tropics where environments are seasonal, but highly unpredictable, allocation decisions may be more sensitive to short-term fluctuations in conditions. We asked (i) whether investments in immune function were affected by inter-annual variation in rainfall and (ii) whether corticosterone and prolactin, two hormones that modulate immune activity in other vertebrates, predict environmentally induced alterations in immune activity in cooperatively breeding superb starlings (Lamprotornis superbus). Superb starlings inhabit African savannas characterized by high among-year variation in rainfall, which influences their breeding life histories and hormone levels. We quantified bactericidal capacity of plasma, or bacterial killing, and prolactin and corticosterone concentrations in blood samples collected over a four year period during the dry season prior to breeding, as this is the period when reproductive roles are determined in this species and when rainfall is most variable. We found that bacterial killing was weakest in the driest year of the study, and we detected a positive relationship between bacterial killing and prolactin, but not a negative relationship with corticosterone. Together these results suggest that prolactin may mediate rainfall-induced changes in immune activity in superb starlings. This study is the first to examine relationships between prolactin and an index of constitutive, innate immunity in birds, and suggests that even species inhabiting unpredictable environments adjust their physiological priorities to environmental conditions, perhaps via prolactin.