Conventional and isotopic determinations of shorebird diets at an inland stopover: The importance of invertebrates and Potamogeton pectinatus tubers

We used gut-content and stable-isotope techniques to determine diets of shorebirds staging at a prairie wetland complex. Stable-isotope ratios for carbon (C-13/C-12) and nitrogen (N-15/N-14) varied greatly within and among prey types and shorebirds, depending on location within the complex. Both dietary techniques suggested that Long-billed Dowitchers (Limnodromus scolopaceus) and Stilt Sandpipers (Calidris himantopus) ate mostly invertebrates, whereas Hudsonian (Limosa haemastica) and Marbled godwits (Limosa fedoa) ate mainly Potamogeton pectinatus tubers. In comparison, the stable-isotope technique indicated that godwits, especially juvenile Marbled Godwits, ate more invertebrates than is indicated by the gut-content analysis. The discrepancies between methods reflect the potential for bias in the application of these techniques. Researchers using stable isotopes to assess migratory shorebird diets should be aware of possible complications arising from isotopic variability within prey types, even over small geographic ranges. High isotopic variability at inland agro-wetland complexes might preclude reliable isotopic assessment of shorebird diets, especially long term. Rhizivory in godwits may be more common than is generally recognized, especially at inland stopovers during autumn migration in both North America and Eurasia, and should be factored into conservation initiatives for these species.