No abstract

Rearing environments can shape offspring phenotype across taxa, yet little is known about how brood size influences hypothalamic–pituitary axis functioning, whether its expression trades off with growth, and the degree to which these relationships vary between species. We evaluated how brood size influenced nestling physiological (glucocorticoids) and somatic traits (growth), and the extent to which their relationship differed between two closely related, sympatric songbirds when experiencing identical rearing environments. Specifically, we used a cross‐fostering approach to alter brood size and create an experimental gradient of nestmate competition, and then tested whether experimentally manipulated brood sizes resulted in nestlings with altered concentrations of corticosterone and whether corticosterone responses traded off with growth. Nestlings of both species experienced elevated concentrations of baseline and stressor‐induced corticosterone when raised in enlarged broods, relative to control and reduced broods, but neither measurement was found to trade off with growth or be linked to survival to fledging. In contrast, we found divergence in the magnitude of the corticosterone stress response between species across all brood treatments, with greater stressor‐induced corticosterone concentrations found in the Violet‐green Swallow (Tachycineta thalassina) relative to the closely related Tree Swallow (T. bicolor). Our study demonstrated that brood size can lead to changes in offspring corticosterone concentrations in swallows and that nestlings of sympatric species, even those that are closely related and ecologically similar, can diverge in their corticosterone stress response when experiencing identical rearing conditions. 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 enhance offspring survival during challenging environmental conditions, such as when brood competition is strong. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13116/suppinfo is available for this article.

To function effectively, animal signals must transmit through the environment to receivers, and signal transmission properties depend on signal form. Here we investigated how the transmission of multiple parts of a well-studied signal, bird song, varies between males and females of one species. We hypothesized that male and female songs would have different transmission properties, reflecting known differences in song form and function. We further hypothesized that two parts of male song used differentially in broadcast singing and aggressive contests would transmit differently. Analyses included male and female songs from 20 pairs of canyon wrens (Catherpes mexicanus) played and re-recorded in species-typical habitat. We found that male song cascades used in broadcast singing propagated farther than female songs, with higher signal-to-noise ratios at distance. In contrast, we demonstrated relatively restricted propagation of the two vocalization types typically used in short-distance aggressive signaling, female songs and male “cheet” notes. Of the three tested signals, male “cheet” notes had the shortest modeled propagation distances. Male and female signals blurred similarly, with variable patterns of excess attenuation. Both male song parts showed more consistent transmission across the duration of the signal than did female songs. Song transmission, thus, varied by sex and reflected signal form and use context. Results support the idea that males and females of the same species can show distinctly different signal evolution trajectories. Sexual and social selection pressures can shape sex-specific signal transmission, even when males and females are communicating in shared physical environments.