Effect of brood size manipulation on offspring physiology: an experiment with passerine birds

The environment experienced during ontogeny has a significant impact on the physiological condition of offspring. This, in turn, forecasts survival probabilities and future reproductive potential. Despite the prominent role that the concept of condition plays in evolutionary studies, the physiological and biochemical characters that define it remain relatively unexplored. In this study, we quantified the impact of brood size manipulations on the physiology and biochemistry of nestling tree swallows (Tachycineta bicolor) shortly before they fledged. Over two breeding seasons, we either increased or decreased the number of individuals in a brood by a single nestling. Every 2-4 days, we determined the resting rate of oxygen consumption [V(O(2))] of individuals in each brood. Growth was followed until 16 days of age, at which time, to look for potential trade-offs in energy allocation, we measured total lipid mass, skeletal muscle and organ mass, indices of blood oxygen-carrying capacity and the activities of key metabolic enzymes in various tissues. Surprisingly, there was a minimal response of most characters to brood manipulation, suggesting that physiological and biochemical development is relatively invariant except perhaps under extreme conditions. Individuals reared in artificially enlarged broods, however, had a significantly lower body mass, body-size-adjusted [V(O(2))], gizzard mass and total lipid mass. These individuals also had decreased activity of cardiac 3-hydroxyacyl CoA dehydrogenase, suggesting a decreased capacity for oxidation of fatty acids. How these characters affect survival or the future adult phenotype remains unknown.