Rapidly growing animals or those that are recovering from nutritional stress may use exogenous nutrients differently from well fed adults. To test this possibility, we compared the rates of exogenous nutrient oxidation among fledgling, fasted adult, and refed adult zebra finches using a technique called breath testing, where animals are fed (13)C-labeled nutrients and (13)C in the exhaled breath is collected and quantified. In order to identify the possible mechanisms responsible for differences in oxidative kinetics of ingested nutrients, we also compared body mass (m(b)), organ mass, core body temperature (T(b)), and metabolic rate (MR). We found that fasted birds had lower T(b), relative liver and intestine masses, MR, and respiratory exchange ratios (RERs) than fed adults. Adult birds recovering from nutritional stress had much lower rates of exogenous nutrient oxidation than fed birds; this difference was particularly evident for fatty acids. Differences in oxidative kinetics were correlated with reduced RER, m(b), and liver mass, suggesting that previously fasted birds were using recently assimilated nutrients to replenish exhausted fuel stores. Rapidly growing fledglings oxidized exogenous nutrients as quickly as fed adults, despite their significantly lower m(b) and T(b). We suggest that fledglings had higher mass-specific rates of exogenous nutrient oxidation because they must compensate for the relatively low conversion efficiency of feather production and other lean tissue growth, which was not taking place in the adults. Although this study demonstrates that ontogeny and nutritional status influence the way that birds oxidize exogenous nutrients, it also underscores the likelihood that environmental and endogenous factors shape how other types of animals spend the nutrients they ingest.