No abstract

Avian diet quality is typically measured using true metabolizable energy (TME N ), which is a measure of assimilable energy of food items accounting for innate endogenous losses. Originally developed for use in the poultry industry, TME N methods have been adapted to determine the value of natural foods consumed by waterfowl to parameterize bioenergetics models for conservation planning. Because there is little knowledge of the variation in TME N estimates among food items and waterfowl species, we investigated TME N of 6 common species of submersed aquatic vegetation for mallards (Anas platyrhynchos; i.e., a diet generalist) and gadwall (Mareca strepera; i.e., a diet specialist) in the midwestern United States during autumn 2015-2017. We precision fed and collected excreta from ducks using standard bioassays to estimate TME N . Mallards had slightly greater TME N than gadwall, but there was considerable variation in TME N among vegetation species, duck species, and individuals within each species. True metabolizable energy (±SE; kcal/g[dry]) for mallards was greatest for Canadian waterweed (Elodea canadensis; 1.66 ± 0.26), followed by coontail (Ceratophyllum demersum; 1.51 ± 0.28), southern naiad (Najas guadalupensis; 1.37 ± 0.39), sago pondweed (Stuckenia pectinata; 0.50 ± 0.22), wild celery (Vallisneria americana; 0.05 ± 0.42), and Eurasian watermilfoil (Myriophyllum spicatum; -0.13 ± 0.42). Mean TME N for gadwall was greatest for Eurasian watermilfoil (0.77 ± 0.32), followed by Canadian waterweed (0.70 ± 0.31), coontail (0.55 ± 0.28), southern naiad (-0.61 ± 0.34), wild celery (-0.98 ± 0.39), and sago pondweed (-1.07 ± 0.33). Generally, TME N for most vegetation species was less than agricultural grains, but it was similar to ranges reported for seeds of naturally occurring hydrophytic vegetation and aquatic macroinvertebrates. We recommend that conservation planners incorporate species-specific TME N estimates in bioenergetics models and that future researchers improve TME N assays for wild waterfowl following our recommendations.

True metabolizable energy (TME) of waterfowl foods is commonly estimated for use in energetic carrying capacity models in association with conservation planning efforts under the North American Waterfowl Management Plan. Researchers define and estimate TME as the net energy from excreta collected from an individual after feeding a precise quantity of a food item compared with excreta collected during a fasted control. We evaluated an alternative excreta collection method for use in TME assays for comparison with traditional excreta collection methodology for waterfowl. We fed widgeongrass (Ruppia maritima) to wild‐caught mallards (Anas platyrhynchos) during autumn 2016 at Forbes Biological Station, Havana, Illinois, USA, and quantitatively collected excreta using a harness‐style excreta collection method and a traditional basin‐style collection method. We were unable to find support for a difference in total excreta energy, mass loss, and TME estimates between collection methods, but high variation in TME among samples prohibited our ability to consider the collection methods similar. Time required for processing harness samples was 60–175 minutes less per sample than for basin samples. The basin method has been the standard for TME trials on waterfowl, but we found the harness method to be much more efficient and potentially allow for better animal husbandry practices. We suggest the harness method be tested further with increased sample sizes or alternate foods prior to widespread application, but researchers conducting TME assays of waterfowl foods should consider using this method in the future. © 2019 The Wildlife Society.