Abstract
The measurement of corticosterone (CORT) levels in feathers has recently become an appealing tool for the conservation toolbox, potentially providing a non-invasive, integrated measure of stress activity throughout the time of feather growth. However, because the mechanism of CORT deposition, storage and stability in feathers is not fully understood, it is unclear how reliable this measure may be, especially when there is an extended interval between growth and feather collection. We compared CORT levels of naturally grown feathers from tree swallows (Tachycineta bicolor) that were moulted and regrown concurrently and therefore expected to have similar CORT levels. Specifically, we compared the same feather from the left and right wing (moulted symmetrically) and different types of feathers (wing, back and tail) expected to have been moulted within the same time period. We found that larger, heavier feathers held more CORT per unit length. In addition, we found a lack of concordance in CORT levels both within the same feather type and between different feather types, even after taking into account differences in feather density. Our results indicate that naturally grown feathers may not consistently provide an indication of stress status. Additionally, conflict in results may arise depending on the feather assayed, and total feather volume may be an important consideration when interpreting feather CORT levels. Future work is necessary to determine explicitly the mechanisms of CORT deposition, the effects of environmental exposure and feather wear on the permanence of the feather CORT signal, and the influence of responses to wild stressors on feather CORT levels, before feather CORT can be implemented effectively as a tool for ecological and conservation applications.
Highlights
The use of physiological measures as biomarkers of environmental change and disturbance in species of conservation importance has been proposed to be a powerful tool for practitioners (Cooke et al, 2013)
We found no relationship between feather CORT levels from the left and right side of a bird, and the coefficient of determination for the model was low, indicating that feathers moulted at the same time do not have the same feather CORT level
Feather CORT levels are often interpreted as indicators of condition or environmental challenge at the location of moult (e.g. Harms et al, 2010; Legagneux et al, 2013), predictors of breeding success or survival (e.g. Koren et al, 2012) or biomarkers of carry-over effects (e.g. Crossin et al, 2013; Bourgeon et al, 2014; Harms et al, 2015)
Summary
The use of physiological measures as biomarkers of environmental change and disturbance in species of conservation importance has been proposed to be a powerful tool for practitioners (Cooke et al, 2013). Glucocorticoid (GC) activity has been suggested as one such biomarker because of the role of GCs in daily energy balance and in response to acutely stressful events Measuring GCs in the circulation can be difficult, invasive and limited to certain time periods; issues that are especially undesirable in a metric directed towards species of conservation concern (Sheriff et al, 2011). Determining the mechanisms of corticosterone (CORT) deposition into feathers, the specificity of assays/ antibodies, hormone stability over time, mass dependency, influences of feather colour and type, and variation in CORT along feather length are all necessary in order to understand fully how to interpret feather CORT levels within and across individuals (Lattin et al, 2011; JenniEiermann et al, 2015; Berk et al, 2016; Romero and Fairhurst, 2016)
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