In this study, we compare the ontogenic patterns for thyroid hormones, growth hormone (GH), and insulin-like growth factor-I (IGF-I) in altricial European starlings and precocial Japanese quail and examine the effects of feed restriction on these species. The most marked difference in development between the altricial and precocial birds was with respect to plasma thyroid hormone patterns. In the starling, circulating concentrations of triiodothyronine (T3) and thyroxine (T4) were very low in embryos, then increased progressively after hatching to peak at 10-11 days of age. In contrast, in quail, in which other studies have shown that most thyroid maturation occurs during the embryonic and peri-hatch periods, the circulating concentrations of T3 and T4 showed little posthatch ontogenic change. Plasma concentrations of both GH and IGF-I showed similar patterns in both species with a posthatch rise (peak at 3 days in starlings and 8 days in quail), followed by a decline. Food restriction to maintain body weight resulted in decreased plasma concentrations of T3 and IGF-I in both species. After return to ad libitum feeding, plasma T3 and IGF-I increased in both early and late restricted starlings and in late restricted quail. Although both species responded to food restriction with similar patterns of endocrine change, age-related differences in the magnitude of hormonal responses were observed.