The file contains biomass and count data of food items encountered in the digestive tract of collected green-winged teal from the Illinois River Valley during spring 2016-2018. The file also contains biomass of food items collected from core samples collected at sites where the green-winged teal were collected. Together, the consumed and availability food data are used to calculate diet selection. The data also contains information on the teal, collection, sites, and other covariates used in analysis. Lastly, the dataset contains biomass of food items collected in medium (#35) and small (#60) sieves for 2018 core samples.