Body Condition of Spring-Migrating Green-Winged Teal

Resource availability during spring migration may limit body condition of dabbling ducks at mid-latitude wetlands used as stopover sites. Body condition can impact stopover length and ability to reach breeding grounds which may affect reproductive effort. We performed proximate analysis on the carcass of 161 green-winged teal (Anas crecca, hereafter teal) collected at active foraging locations in the Illinois River Valley (IRV) during February–April 2016–2018. We evaluated the effects of extrinsic (e.g., wetland location and connectivity, timing, weather, river level, food availability) and intrinsic (diet composition, sex, age) factors on carcass lipid and protein levels using linear mixed models. Additionally, we used morphometric measures of collected teal to evaluate the accuracy of three commonly used non-destructive condition indices; the scaled mass index, scaled wing index, and body-size index. Carcass lipids were 18.3% (95% CI = 1.0–38.5) and 21.6% (4.8–41.0) greater at collection sites with moderate (250–600 kg/ha) and high (>600 kg/ha) moist-soil seed densities, respectively, than low densities (<250 kg/ha) sites. Lipids also increased 2.4% (0.2–5.1) with every 10% increase in the proportion of plant seeds in diets. Carcass fat decreased 0.9% (0.4–1.4) and 3.8% (2.1–5.4) with every 1-day delay in collection date and 1 C rise in temperature, respectively. Non-destructive condition indices were poor (r ≤0.45) estimators of carcass lipid content, and we suggest researchers develop and further evaluate indices before broad application across species. While stopover duration and arrival time in the IRV likely affected body condition, the availability of quality seasonal-emergent wetlands with ample food resources may be most critical to teal body condition during spring migration.