Plasma metabolite indices are robust to extrinsic variation and useful indicators of foraging habitat quality in Lesser Scaup | Los índices de metabolitos plasmáticos son robustos a la variación extrínseca y son indicadores útiles de la calidad del hábitat de forrajeo en Aythya affinis

Energy acquisition and storage are important for survival and fecundity of birds during resource-limited periods such as spring migration. Plasma-lipid metabolites (i.e. triglyceride [TRIG], β-hydroxybutyrate [BOHB]) have been used to index changes in lipid stores and, thus, have utility for assessing foraging habitat quality during migration. However, such an index may be affected by energetic maintenance costs, diet, and other factors, and further validation under experimental conditions is needed to understand potential sources of variation and verify existing indices. We evaluated a plasma-lipid metabolite index using 30 female and 28 male wild Lesser Scaup (Aythya affinis; hereafter scaup) held in short-term captivity (∼24 hr) during spring migration. Similar to previous observational studies, BOHB was negatively associated and TRIG was positively associated with mass change (R² = 0.68). BOHB estimates were nearly identical to those published on free-living scaup, but TRIG estimates differed from free-living scaup and varied by sex, with females having a greater rate of predicted mass change than captive and free-living males. Our results suggest TRIG may be a better measure of energy income than deposition because lipid deposition likely varies with energetic maintenance costs, stress, and underlying physiological processes while TRIG relates primarily to energy income. In contrast, BOHB was a reliable predictor of negative mass change across sexes. The sex-based differences in apparent lipid deposition rates warrant further research before a generalizable model is advisable for comparing mass change predictions across studies. However, if predictions are standardized, this technique is generally robust to variations in energy income vs. lipid deposition across sexes. Accordingly, our evaluation provides verification for the utility of plasma-lipid metabolites as an indicator of foraging habitat quality during migration. LAY SUMMARY The past several decades have seen a growing body of literature on the utility of plasma-lipid metabolites to assess habitat quality for a multitude of migratory bird species. Research in this area is often focused on 2 key metabolites, β-hydroxybutyrate and triglyceride and their associated relationships with mass change. Triglyceride has a positive relationship with mass change and is often referred to as an indicator of lipid deposition or fattening rates. However, energetic maintenance costs may confound the relationship of triglycerides and lipid deposition. To the best of our knowledge, no studies have attempted to control for potential variation in energy expenditure in conjunction with metabolite sampling during migration. Thus, we believe our paper will be a useful addition to the scientific literature on these metabolites and will be of interest to a broad audience of national and international wildlife managers and avian researchers.