Welcome to the American Ornithological Society 2018 Annual Conference. We are pleased to have you join us at the lovely Hilton El Conquistador Resort in Tucson, AZ.
Rearing environments can shape offspring phenotype across taxa and depend on the types and magnitude of stressors young experience during development. However, little is known about how brood size, a key component of rearing environment for many taxa, influences the way in which the physiological stress response develops, and how development may vary between sympatric, closely related species. Using free-living, box-nesting populations of Tree Swallows (Tachycineta bicolor) and Violet-green Swallows (T. thalassina), we evaluated how brood size influenced stress axis functioning, growth rates, and survival of nestlings. Specifically, we tested whether experimentally manipulated brood sizes resulted in changes to corticosterone concentrations in offspring, if changes in corticosterone traded off with growth, and how these measures varied between two related species when they experienced identical rearing environments. Nestlings of both species experienced elevated concentrations of corticosterone when raised in enlarged broods relative to control and reduced broods, but neither measurement traded off with growth or was linked to survival. Against our predictions, we found marked divergence in the magnitude of the corticosterone stress response between species, with greater stressor-induced corticosterone concentrations in the Violet-green Swallow. Our study showed that corticosterone responses can vary between even closely related species experiencing the same environments. We conclude that corticosterone appears to play a key role for balancing energetic demands that arise in the face of nestmate competition in Tachycineta swallows, and that elevated concentrations of corticosterone may sustain offspring survival during challenging environmental conditions, such as when brood competition is high.
