Ontogeny of immune function in altricial birds has been understudied. We examined development of the innate immune system in free-living house sparrows and the flexibility of innate immune response to food restriction (FR) and refeeding in captive house sparrows. Ecological immunology techniques were used to examine components of the innate immune system. Natural antibody agglutination and complement-mediated lysis increased with age, yet levels in near-fledgling birds were about 50% lower than in hatch-year birds and adults. Lysozyme levels declined with age, reflecting catabolism of maternal lysozyme transferred via the egg. In captive house sparrows, FR did not significantly impact lysis, a constitutive component of innate immunity, yet levels of the acute-phase protein haptoglobin, an inducible component of innate immunity, were reduced in FR birds. Birds refed after early FR showed no reduction in haptoglobin response. These findings indicate that significant maturation of the immune system occurs post-fledging in altricial house sparrows, and may challenge the notion that juveniles depend primarily on innate immune defenses during the growth period. Also, innate immune function appears to be flexible to resource supply during the nestling period, with no evidence of long-term effects of early FR on development of the innate immune system. Support: NSF-GRF, AOU, USDA-Hatch, Birge Fund, GWIS.