- 1. Early competitive environments can shape the development of offspring across taxa. For example, competition for food can have downstream effects on offspring phenotypes and fitness; however, little is known about how early competition influences the development of the hypothalamic-pituitary axis and its potential tradeoff with growth. Because baseline and stressor-induced concentrations of glucocorticoids serve different purposes yet are central to survival, both should be evaluated both when assessing these potential tradeoffs.
2. We used a reciprocal partial cross-fostering study design in a free-living population of two species of sympatric, closely related bird species to test the hypothesis that increased competition during post-natal development leads to tradeoffs in the production of corticosterone (CORT), the primary glucocorticoid in birds, and growth. In addition, we tested the extent to which these responses differed between closely related species experiencing the same experimental conditions.
3. We found that nestlings of both species had elevated concentrations of both baseline and stressor-induced CORT in enlarged broods relative to control and reduced broods. In addition, we detected higher stressor-induced CORT in Violet-green Swallows (Tachycineta thalassina) when compared to Tree Swallows (T. bicolor) across treatments.
4. Violet-green Swallows grew slower than Tree Swallows in all treatments, and growth rate tended to decrease as brood size increased in both species. Further, we did not detect a treatment effect on the offspring survivorship to fledge or length of nesting period. Thus, elevated levels of CORT appear to be adaptive via changes that maintain nest survival rates under challenging conditions, such as when brood competition is high.
5. Our study showed that nestlings of closely related species that experience the same environmental stressors can diverge in their CORT response, indicating that species-level variation in CORT expression may be more important than environmental drivers. Further, we found that variation in early competition can lead to changes in CORT expression in swallows, and that this expression trades off with growth when brood competition is high. CORT may play a key role for balancing energetic demands that arise in the face of nestmate competition.
Key Words: corticosterone, glucocorticoids, growth, Tachycineta, tradeoffs
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