Comparing four indices of physiological stress in an urban-adapted songbird between urban and rural habitats

Birds show tremendous variation in the extent to which different species tolerate life in urban habitats, yet it remains unclear why some species thrive while others cannot. My work explores possible phenotypes that make an urban-adapted species, the European starling (Sturnus vulgaris), so successful in both rural and urban areas. Literature on stress-urbanization relationship remains highly equivocal partly due to an array of different study systems being compared as well as to key methodological differences. The present study quantified metrics of both acute stress (i.e., stress-induced corticosterone and breath rate) and chronic stress (i.e., baseline corticosterone and total leukocyte count) in two rural and two more urban starling populations to better understand the relationship between stress and urbanization. Our principal finding was that urban adult starlings had significantly higher levels of stress-induced corticosterone than rural conspecifics. We found no difference between urban and rural populations in baseline corticosterone, breath rate, or total leukocyte count. We found that baseline corticosterone and body condition were negatively correlated, while other traits did not covary with body condition. Understanding endocrine responses to habitat urbanization and anthropogenic disturbances is a crucial step in finding a well-validated protocol for determining whether an individual or a population is, in fact, experiencing stressful conditions and to what degree they are capable of quickly and strongly mounting a potentially life-saving stress response. Incorporating multiple indices of stress is of equal importance as these metrics may represent different aspects of the organismal stress response, potentially being affected independently by endogenous or exogenous drivers