Not absolutely sure Luie but I can venture an estimated guess based on 50 yrs. of duck hunting. Mallards are true only true grain field species. So when they leave the river or refuges to feed around here they usually go to the corn fields. That's when we get to shoot them trading back and forth between feed, water and rest areas. Pintails, Gaddies, Teal, are tru "dabblers". Although they will also feed in grain fields, especially after a freeze up, they do as the name implies. Millet, hogweed, duckweed, Walters millet, smartweed and all the natural moist soil plants that grow in shallow water / mud flats make up the balance of their diet. Also by the time we get to good hard freezes and the mallards start working the "freeze line" the rest of the ducks go on south. Pintails and Widgeon are open shallow water ducks. They don't like confined spaces to begin with. I hunted Pintails on the west coast a lot when I was in the Navy years ago. Their tactics amazed me then as they do now. Hope this helps and it's not from an expert, just a lot of "duck days" under my belt. Any Wildlife / Waterfowl biologists out there please chime in. Excellent question coming from an interested young man.