Innate immune functions are proposed to develop rapidly post-hatch in altricial nestlings, compared with adaptive immune defenses that require development of receptor specificity and memory. Studies of ontogenetic changes in altricial birds have been few until relatively recently and often do not encompass the entire developmental period. We examined the patterns of development in constitutive innate and adaptive immune indices in house sparrow nestlings (3, 6, 9 and 12 days (d) post-hatch), hatch-year birds and adults. Lysozyme activity significantly decreased with age, likely representing catabolism of maternal investment of lysozyme in the egg albumen. Levels of total IgY (indexing adaptive immune function), as well as agglutination and lysis (indexing innate immune function), increased throughout the nestling period, but were significantly below levels found in fully-grown birds at the time of fledging. There were no significant differences between hatch-year birds and adults in these measures, indicating that rapid, full maturation occurs early in the post-fledging period. In combination with previous studies, these data highlight the importance of sampling fledglings to assess full immune ontogeny and suggest that fledgling birds may be more vulnerable to infection than adults.