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

Abstract

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.

The prospects of poop: a review of past achievements and future possibilities in faecal isotope analysis

“…This could be driven by the fact that a significant proportion of the nitrogen found in mammal faeces is sourced from bacterial and endogenous debris, as discussed in Section II.1. Perhaps faecal samples also contain isotopically fractionated nitrogenous metabolites that could be removed using a pretreatment procedure, as advocated by Kuwae et al (2022).…”

“…Should certain dietary items, such as masting fruits, gain importance seasonally, then the amount of time represented by a faecal sample may also vary seasonally. Furthermore, faeces that are passed quickly may contain a larger proportion of undigested materials (and therefore a smaller proportion of gut tissue and epithelial cells), thereby reducing the magnitude of Δ diet-faeces values, as suggested by Kuwae et al (2022) for shorebirds. This potential variability in diet-faeces discrimination with diet is important for researchers to keep in mind when comparing faecal isotope data across seasons.…”

“…Other authors have additionally used chemical pretreatments, such as acidification to remove carbonates (e.g. Reid & Koch, 2017; Kuwae et al ., 2022), and/or treatment with solvents such as chloroform: methanol or sodium hydroxide (NaOH) to remove isotopically fractionated metabolites like urea and ammonium (e.g. Wurster et al ., 2017; Kuwae et al ., 2022).…”

“…Reid & Koch, 2017; Kuwae et al ., 2022), and/or treatment with solvents such as chloroform: methanol or sodium hydroxide (NaOH) to remove isotopically fractionated metabolites like urea and ammonium (e.g. Wurster et al ., 2017; Kuwae et al ., 2022). In bat and seabird guano accumulations, ammonia volatilisation has been shown to increase guano δ 15 N values significantly (e.g.…”

“…(2016); 4 = Varo & Amat (2008); 5 = Kuwae et al . (2022); 6 = Tsahar et al . (2008); 7 = Jones et al .…”