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 o f shorebirds staging at a prairie wetland complex. Stable-isotope rati os for carbon (C-13/C-12) and nitrogen (N-15/N-14) varied greatly with in and among prey types and shorebirds, depending on location within t he complex. Both dietary techniques suggested that Long-billed Dowitch ers (Limnodromus scolopaceus) and Stilt Sandpipers (Calidris himantopu s) ate mostly invertebrates, whereas Hudsonian (Limosa haemastica) and Marbled godwits (Limosa fedoa) ate mainly Potamogeton pectinatus tube rs. 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 metho ds reflect the potential for bias in the application of these techniqu es. Researchers using stable isotopes to assess migratory shorebird di ets should be aware of possible complications arising from isotopic va riability within prey types, even over small geographic ranges. High i sotopic variability at inland agro-wetland complexes might preclude re liable isotopic assessment of shorebird diets, especially long term. R hizivory in godwits may be more common than is generally recognized, e specially at inland stopovers during autumn migration in both North Am erica and Eurasia, and should be factored into conservation initiative s for these species.