Substantial development of the immune system occurs during the nestling period, yet the resource dependence of this immune development is understudied. We examined constitutive immune system development as well as acute-phase responses to lipopolysaccharide (LPS) injection after 48 h of food restriction in house sparrows at 7 and 13 d posthatch. We also examined nestlings that were restricted early (5-7 d) but refed and tested at 13 d posthatch to determine whether altered immune function and growth early in the nestling period were recovered upon return to adequate resource supply. Induced acute-phase protein response was reduced in food-restricted birds, yet no lasting reductions in acute-phase protein levels were observed in previously restricted nestlings that were challenged with LPS after refeeding. Food restriction did not significantly impact constitutive levels of complement-mediated lysis or circulating IgY antibodies. As a comparator to immune measures, we found that organ and tarsus size, as well as muscle size and citrate synthase enzyme activity (an index of muscle cellular aerobic capacity), were significantly reduced in food-restricted nestlings. Reductions in flight muscle mass and function persisted in birds refed after early food restriction, which may have contributed to persistent body temperature reductions observed in refed birds.