Variation in True Metabolizable Energy Among Aquatic Vegetation and Ducks

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.

Human access constrains optimal foraging and habitat availability in an avian generalist

“…Our fifth and sixth wetland categories were derived from a regional waterfowl‐focused land cover layer of unharvested flooded agriculture and intensively managed emergent wetlands, which were hand‐digitized and combined with our CDL base layer (Highway, 2022; Masto, 2023; Palumbo et al, 2019). Final wetland classifications included six wetland types that were presumed to generally increase in energetic quality based on extensive published literature (i.e., open water < woody wetlands < emergent wetlands < managed emergent wetlands < harvested agriculture < unharvested agriculture; Foster et al, 2010; Gross et al, 2020; Hagy et al, 2011; Hagy & Kaminski, 2012; Kaminski et al, 2003; Osborn et al, 2017; Tapp et al, 2018; Table 1; Appendix S1). However, we did not assign energy values to our categorical wetland variables explicitly; instead, we assumed their energetic quality, in a similar fashion to North American conservation planning initiatives and their partners (Edwards et al, 2012; Hagy et al, 2021; Hagy, Wirwa, Hitchcock, 2021; Soulliere et al, 2017).…”

Energy Values of Corn and Rice Bran and Energy Levels for Ducks – Basis in Establishing Energy Requirement for Improved Philippine Mallard Duck

“…As proof, Lancaster et al, (2018) showed that bioavailable energy particularly total metabolizable energy (TME) of submersed aquatic vegetation, was highly variable among duck species, favoring mallard ducks compared with gadwall (Mareca Strepera). A recent study demonstrated a similar result that mallards had slightly greater nitrogen-corrected total metabolizable energy (TME N ) than gadwall (Mareca Strepera) for submersed aquatic vegetation species such as 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) (Gross et al, 2020). Likewise, the degree of growth is also a factor that influences energy metabolism and consequent utilization.…”

Effects of submerged aquatic vegetation and water quality on waterfowl abundance by foraging guild

“…In addressing the unexpected tendency of certain guilds to decrease despite more abundant SAV, many interviewees noted the importance of vegetative community composition and its relationship to waterfowl foraging behaviour. For instance, assimilable energy may provide insight into the vegetation valued by waterfowl species within each guild (Dugger et al, 2007;Fredrickson & Reid, 1988;Gross et al, 2020). Southern naiad (Najas guadalupensis), wild celery (Vallisneria americana), coontail (Ceratophyllum demersum), sago pondweed (Potamogeton pectinatus), and Eurasian watermilfoil (Myriophyllum spicatum) are all important waterfowl resources found in Back Bay with varying degrees of tolerance to turbidity and salinity (Carter et al, 1983;Carter & Rybicki, 1991;Chamberlain, 1948;Tiner & Rorer, 1993) and varying degrees of true metabolizable energy for waterfowl (Gross et al, 2020).…”

“…For instance, assimilable energy may provide insight into the vegetation valued by waterfowl species within each guild (Dugger et al, 2007;Fredrickson & Reid, 1988;Gross et al, 2020). Southern naiad (Najas guadalupensis), wild celery (Vallisneria americana), coontail (Ceratophyllum demersum), sago pondweed (Potamogeton pectinatus), and Eurasian watermilfoil (Myriophyllum spicatum) are all important waterfowl resources found in Back Bay with varying degrees of tolerance to turbidity and salinity (Carter et al, 1983;Carter & Rybicki, 1991;Chamberlain, 1948;Tiner & Rorer, 1993) and varying degrees of true metabolizable energy for waterfowl (Gross et al, 2020). Moreover, diving ducks such as bufflehead (B. albeola) and mergansers (hooded [L. cucullatus] and red-breasted [Mergus serrator]) may be more tolerant of higher salinity conditions due to their ability to dive for marine crustaceans and molluscs (Perry et al, 2007).…”

“…As a vital primary producer in both aquatic and adjoining upland habitats, SAV provides a range of ecosystem services including water column filtration, sediment stabilization, and biodiversity support through habitat and food resource provision (de Boer, 2007; Hestir et al., 2016; Madsen et al., 2001; Munro & Perry, 1983; Stevenson & Confer, 1978). In Back Bay, Virginia, SAV has historically supported large populations of waterfowl, including dabbling ducks, like the American black duck ( Anas rubripes ), gadwall ( Mareca strepera ), and mallard ( Anas platyrhynchos ); diving ducks, such as bufflehead ( Bucephala albeola ) and hooded merganser ( Lophodytes cucullatus ); and swans and geese, including Canada goose ( Branta canadensis ), snow goose ( Anser caerulescens ), and tundra swan ( Cygnus columbianus ) (Gross et al., 2020; Morton & Kane, 1994; Perry et al., 2007; Settle & Schwab, 1991; Waterfield, 1951). Throughout the twentieth century, however, fluctuations in the percent frequency of aquatic vegetation in Back Bay (from 87% in 1974 to less than 1% in 1991), and the seemingly associated decline in waterfowl abundance (over 90 thousand in 1977 to less than 10 thousand in 1990) has prompted concern from natural resource managers throughout the region (Orth & Moore, 1983; Schwab et al., 1991; Morton & Kane, 1994; Moorman et al., 2017; Figure S1 in the Supporting Information).…”