Using stable isotope (δ¹³C, δ¹⁵N) values from feces and breath to infer shorebird diets

The use of stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N) from feces and breath offers potential as non-destructive tools to assess diets and nutrition. How stable isotope values derived from breath and feces compare with those from commonly used tissues, such as blood fractions and liver, remains uncertain, including understanding the metabolic routing of dietary nutrients. Here, we measured δ¹³C and δ¹⁵N from feces and δ¹³C of breath from captive Red-necked Stints (Calidris ruficollis) and 26 species of wild-caught migratory shorebirds (n = 259 individuals) and compared them against isotopic values from blood and feathers. For captive birds fed either cereal- or fish-based diets, differences in δ¹³C between feces and lipid-free diet were small, - 0.2 ± 0.5‰ and 0.1 ± 0.3‰, respectively, and differences in δ¹⁵N, - 0.7 ± 0.5‰ and - 0.5 ± 0.5‰, respectively. Hence, δ¹³C and δ¹⁵N values from feces can serve as proxies for ingested proteinaceous tissues and non-soluble carbohydrates because isotopic discrimination can be considered negligible. Stable isotope values in plasma and feces were strongly correlated in wild-caught shorebirds, indicating feces can be used to infer assimilated macronutrients. Breath δ¹³C was 1.6 ± 0.8‰ to 5.6 ± 1.2‰ lower than bulk food sources, and breath C derived from lipids was estimated at 47.5% (cereal) to 96.1% (fish), likely underlining the importance of dietary lipids for metabolism. The findings validate the use of stable isotope values of feces and breath in isotopic assays to better understand the dietary needs of shorebirds.