Food selection by spring-migrating green-winged teal

The Mississippi Flyway supports millions of waterfowl during spring migration as individuals replenish vital nutrients en route to breeding locations.  Green-winged teal Anas crecca consume natural plant and animal foods in shallow wetlands during autumn and winter, but little information is available to describe food use and selection during spring migration.  We experimentally collected green-winged teal and quantified food use and availability to determine selection in the Illinois River Valley during February–April 2016–2018.  We removed, dried, and weighed (± 0.1 mg) food items by taxa from the upper digestive tract (proventriculus and esophagus) of birds and core samples for comparison.  Additionally, we evaluated retention of common diet items between small (#60; 250 µm) and medium (#35; 500 µm) sieves and the effect of processing sieve size on selection coefficients.  Seeds of moist-soil vegetation occurred in all green-winged teal diets, while invertebrates and vegetative material occurred in 67.4% and 25.8% of diets, respectively.  Green-winged teal consumed 85.8% (CI95 = 81.2–90.3%) plant material and 14.2% (CI95 = 9.6–18.7%) invertebrates based on aggregate dry biomass.  We failed to find support for selection of either plant or animal foods in general, but green-winged teal selected Cyperus spp., Ammannia spp., Leptochloa spp., and Potamogeton spp. and avoided Amaranthus spp., Ipomoea spp., Echinochloa spp., and Oligochaeta individual taxa.  We found no support for a difference in selection ratios between sexes, but selection ratios differed among years and wetland connectivity regimes with changes in food availability.  Sieve size had minimal impact on rank and selection intensity of most food items, but only small sieves captured Ammannia spp., which was an important diet item.  We found no evidence that green-winged teal selected invertebrates in our mid-latitude migration study area as has been speculated for dabbling ducks in general during spring migration (i.e., spring diet-shift hypothesis).  We encourage managers to provide shallowly flooded wetlands with desirable plant taxa (e.g., Cyperus spp., Ammannia spp., Leptochloa spp.) for green-winged teal by maintaining actively-managed moist-soil wetlands that are made available during spring migration.