Stressors encountered during the normal developmental period may affect an individual’s phenotype, including immunocompetence, growth, and feather quality. I examined effects of chronic, low-level stress on American kestrel (Falco sparverius) nestlings. Continuous release of corticosterone, a hormone involved in the stress response, can model chronic stress in birds. I implanted males with either corticosterone implants or shams and measured their growth, immune function, and feather coloration. I found no significant differences between groups at the end of the exposure period in either morphometrics (mass, tarsus, wing length, asymmetry, and body condition) or immunocompetence (cell-mediated immunity and heterophil/lymphocyte ratio). Additionally, treatment did not affect humoral immunity or feather coloration. One week subsequent to implantation removal, however, previously undetected differences existed. Sham-implanted birds had significantly longer wings while corticosterone-implanted birds had an increased level of cell-mediated immunity. Therefore, it seems that small increases in basal corticosterone levels have small, but measurable, effects on avian development.