State-Dependent and Social Modulation of Circulating Glucocorticoids in a Nomadic Songbird, the Red Crossbill ( <i>Loxia Curvirostra</i> )

Glucocorticoids facilitate the integration of environmental information and coordination of organismal responses to perturbations. Circulating glucocorticoids are hypothesized to depend on an individual's environment and condition (i.e., state) to facilitate surviving challenges while minimizing fitness costs. Studies specifically focused on sources of individual variation in circulating glucocorticoids are critical to understanding state-dependent modulation of glucocorticoids and integrated phenotypes more broadly. Such studies can also provide insight into the evolution and adaptive significance of circulating glucocorticoids. Here, we repeatedly sample individuals before and during food restriction to identify how and when food availability and intrinsic differences (i.e., body condition and telomere length), including those of social partners, covary with glucocorticoids in captive Red Crossbills (<i>Loxia curvirostra</i>), a nomadic songbird that specializes on foraging for conifer seeds. Conifer seeds are ephemeral resources produced during unpredictable, but locally synchronous, masting events. Fluctuating food availability and social cues, change the behavior and glucocorticoid physiology of Red Crossbills. Pairs consisting of an adult and juvenile were food restricted using an environmental manipulation known to induce socially mediated changes in glucocorticoid signaling. Baseline and stress-induced glucocorticoids were measured before and during food restriction. Amongst adults, stress-induced glucocorticoids declined following food restriction and positively covaried with telomere length, independent of food availability. These results support the hypothesis that the acute glucocorticoid response is adaptively modulated based on environmental conditions and individual differences in state as measured by telomere length. Under food restriction, juvenile baseline glucocorticoids negatively covaried with body condition and the telomere lengths of adult social partners. The covariation between adult telomere lengths and juvenile baseline glucocorticoids suggests that telomere lengths of adults may relate to adult phenotypes, a hypothesis supported by the covariation between adult telomeres and stress-induced glucocorticoids. Further, as patterns were absent before food restriction, our results demonstrate how environmental challenges can reveal the importance of intrinsic differences to organismal responses and social cues. This study leverages a non-model organism experiencing an ecologically relevant environmental challenge to exemplify how intrinsic differences, including those of social partners, can modulate an endocrine mediator of organismal responses to environmental perturbations.