Immune function development has been studied extensively in poultry, yet analogous studies of altricial birds have been few until relatively recently. Altricial birds have the fastest growth rates among animals and some have such rapid development that the time from onset of egg incubation to fledging is less than 4 weeks. This raises questions about the vulnerability of small nestlings to infection, given that at least 4 weeks may be required to develop B-cell lineages with functional Igs against distinct antigens. We predicted that antibody responsiveness in altricial nestlings increases during the post-hatch period and reaches adult levels by fledging age (day 18-20 post-hatch in zebra finches), and that nestlings generate more robust antibody responses to larger injected antigens. Nestling zebra finches were injected at 7 days (7d group) or 14 days (14d group) post-hatch with keyhole limpet hemocyanin (KLH; antigen size ~400kDa) and West Nile Virus (WNV; antigen size ~50kDa) antigens, and boosted 7 days later. Adults were vaccinated in the same manner. Induced KLH- and WNV-specific IgG response was measured using ELISA. Adaptive IgG response to KLH boost significantly increased between the 7d and 14d nestling groups. However, there was no significant difference in adaptive IgG response to WNV boost between 7d and 14d groups. Adult adaptive IgG responses to KLH and WNV were at least 7 times higher than in nestlings, indicating that immunological competence continues to develop during the nestling and perhaps post-fledging periods. Lastly, adaptive IgG response to KLH was higher compared to WNV regardless of age, possibly indicating an important impact of antigen size on detection of antibody response. Support: NSF-GRFP, USDA-Hatch, AOU, GWIS, UW-Noland Fund.