Feather corticosterone (CORT) levels are increasingly employed as biomarkers of environmental stress. However, it is unclear if feather CORT levels reflect stress and/or workload in the wild. We investigated whether feather CORT represents a biomarker of environmental stress and reproductive effort in tree swallows (Tachycineta bicolor). Specifically, we examined whether individual state and investment during reproduction could predict feather CORT levels in subsequently moulted feathers and whether those levels could predict future survival and reproductive success. Through a manipulation of flight cost during breeding, we also investigated whether an increase in stress level would be reflected in subsequently grown feathers, and whether those levels could predict future success. We found that CORT levels of feathers grown during moult did not (1) reflect past breeding experience (n = 29), (2) predict reproductive output (n = 18), or (3) respond to a manipulation of flight effort during reproduction (10 experimental, 14 control females). While higher feather CORT levels predicted higher return rate (a proxy for survival), they did so only in the manipulated group (n = 36), and this relationship was opposite to expected. Overall, our results add to the mixed literature reporting that feather CORT levels can be positively, negatively, or not related to proxies of within-season and longer-term fitness (i.e., carryover effects). In addition, our results indicate that CORT levels or disturbances experienced during one time (e.g., breeding) may not carry over to subsequent stages (e.g., moult). We, therefore, petition for directed research investigating whether feather CORT represents exposure to chronic stress in feathers grown during moult.
fCORT Feather clipping Glucocorticoid Moult Tree swallow
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We thank Ruthven Park National Historic Site for lodging and access to their study site and nestboxes. We also thank the Grand River Conservation Authority and Habitat Haldimand for access to their study site and nestboxes. We thank Peter Marier for his assistance processing feather samples, and Rick Ludkin, Peter Marier, and Nancy Furber for their help with field collection of samples and data. We also thank two anonymous reviewers for improving the manuscript.
Author contribution statement
CMH conceived and designed the experiments, with input from CLM and OPL. CMH and CLM collected and analyzed the data. CMH wrote the manuscript; CLM and OPL provided editorial advice.
Compliance with ethical standards
This study was funded by an Ontario Graduate Scholarship and an NSERC PGS-D (427552) to CLM and Canada Foundation for Innovation (29401), Canada Research Chair (30342), and an NSERC Discovery Grant (478021) to OPL.
Conflict of interest
The authors declare that they have no conflict of interest.
All applicable institutional and/or national guidelines for the care and use of animals were followed (see “Materials and methods” section).
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