A subclinical dose of broad-spectrum antibiotics (Bactrim and Clavarnox) we re given to one-half of nestling Tree Swallow (Tacbycineta bicolor) brood s on nestling day (ND) 3 and 4 while the other half of the brood received a placebo. Cloacal swabs were taken on ND5 and D15 to monitor changes in the gastrointestinal microbial community. Physical measures to track growth and development of nestlings were taken on ND5, ND15, and ND19 . Bacterial counts were low on ND5 in both treated and untreated birds. By ND15 the only component of the microbial community that was altered was the number of bacteria growing on Levine Eosin Methylene Blue agar (EMB) medium, which was higher in the treated individuals. Treated nestlings initially grew slower after antibiotic treatment but rebounded after ND12, so that no difference was detected at ND19 . Some other physical measures were retarded in treated birds, but by ND19 no difference between treated and untreated birds was detected except for shorter keel length in treated birds . These results suggest that the development of nestling gastrointestinal microbiota affects general measures of physical growth and development. Pro longed and targeted antibiotic therapy is predicted to produce specific effects on growth and development.