True metabolizable energy of foods consumed by lesser scaup (Aythya affinis)

The energy derived from available foods is an important factor used in conservation planning for migratory species. Estimating true metabolizable energy (TME) of available foods has become a common method for resource managers to increase reliability in energetic carrying-capacity estimates. Lesser scaup (Aythya affinis; hereafter scaup), have experienced a population decline concurrent with suspected decreases in foraging habitat quality and quantity at spring stopover sites in the upper Midwest, USA. Unfortunately, few TME estimates are available for common diet items of scaup. We estimated nitrogen-adjusted TME (TME_N) of 5 common foods of scaup by conducting feeding trials on wild females and males. True metabolizable energy varied by food taxa, but not by pretrial body mass or sex. Mean TME_N (kcal/g[dry] ± SE) was greatest for wild millet (Echinochloa crus-galli; 2.20 ± 0.14), followed by chironomids (Chironomus spp.; 1.41 ± 0.49), amphipods (Gammarus spp.; 1.33 ± 0.23), planorbid snails (Planorbidae; 0.17 ± 0.07), and fingernail clams (Sphaeriidae; −0.79 ± 0.27). Our results, combined with scaup diet literature indicated that the management of spring staging areas for high-energy invertebrates (i.e., chironomids and amphipods) would provide improved opportunity for energy acquisition during migration. Further study could help determine if the acclimation of scaup to particular diets, especially bivalves, increases their TME_N values.