Noninvasive assessment of corticosterone and triiodothyronine levels in the endangered Pyrenean Capercaillie (Tetrao urogallus aquitanicus)

Endocrine regulation and hormonal responses determine the capacity of an organism to cope with changing environmental conditions, such as variable or extreme temperatures. Hormones can also be regarded as “sensors” for environmental signals and as indicators of an organism's or populations' fitness. The levels of glucocorticoid hormones, such as corticosterone, are generally used as biomarkers for physiological stress and metabolic rate variations, while thyroid hormones (i.e., triiodothyronine—T3) are regarded as indicators of nutritional and thermal stress in birds. The Pyrenean Capercaillie ( Tetrao urogallus aquitanicus ) is an endangered galliform adapted to cold weather, and its population is in steep decline. This study used a noninvasive approach to assess hormone levels in the Pyrenean Capercaillie, in a first attempt to understand how hormone levels may respond to certain environmental variables. We determined the corticosterone and T3 levels in 139 freshly molted covert feathers of 113 Pyrenean Capercaillies (80 from Spain and 33 from France) using enzyme immunoassays. The relationships between hormone levels and several biotic and abiotic variables were assessed. Our findings showed that corticosterone levels were lower in feathers found further away from areas of human leisure activity (i.e., cycling trails). Lower corticosterone levels were also found in certain biogeographical areas (i.e., Northern Pyrenees and Pre‐Pyrenees), suggesting that corticosterone levels may respond to certain climate‐related variables (such as precipitation or temperature). Conversely, T3 levels were influenced by sex and feather length, but did not correlate with any of the environmental variables. The lack of statistically significant differences in corticosterone and T3 in relation to the types of covert feathers analyzed facilitates the collection process for noninvasive studies. Additional hormonal studies could provide essential data to understand the level of endocrine flexibility of the Pyrenean Capercaillie in response to external variables, which may ultimately define the species' resilience to climate change and other drivers of environmental variation.