Assessing baseline glucocorticoids as conservation biomarkers in a declining aerial insectivore

Conservation biologists are increasingly incorporating a diversity of integrative approaches to monitor, manage, and mitigate the growing threats to biodiversity imparted by climate change and other anthropogenic pressures. Over the past 15 years, stress hormones (i.e., glucocorticoids: corticosterone and cortisol) have been gaining considerable attention as sensitive physiological biomarkers of wildlife disturbance. However, despite a substantial accumulation of studies citing glucocorticoids (GCs) as potential indicators of condition, health, or disturbance, comparatively little is known about their actual utility for conservation monitoring. This thesis aims to validate three key characteristics of baseline plasma GCs that are necessary to their employment as sensitive, predictive biomarkers of wildlife disturbance: 1) correlation with environmental quality; 2) consistency across individuals in response to environmental alteration; 3) relationship with fitness metrics at the individual and population level. I complete these validations across two different reproductive stages in female tree swallows (Tachycineta bicolor), a member of the aerial insectivore guild of birds that is in population decline in North America. My results indicate that baseline GCs may not reflect the natural variation in components of the internal and extrinsic environment that are associated with habitat quality or disturbance. In addition, baseline GCs show considerable within-individual variation across the breeding season, and display individually-specific responses to an experimentally-induced change in environmental quality (i.e., a decline in foraging profitability). Further, baseline GC levels do not relate to multiple metrics of fitness (offspring quality, reproductive output, or survival) despite the careful control of potentially confounding contexts such as age, reproductive stage, time of day, and body condition. Finally, at the average level, my results indicate that an environmental perturbation (i.e., a decline in foraging profitability) can have consequences for body condition, behaviour, and current and future baseline GC levels in habitat type-specific ways without concomitant influences on fitness. Collectively, my findings suggest that baseline GCs may not be easily interpretable as individual or population-level indicators of disturbance or fitness. Importantly, these results indicate that GCs cannot be assumed to represent conservation biomarkers across species or time periods without careful validation.

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