With the first freeze-up, mallards typically appear by thousands. They cover the remnants of open water or sit on the ice. If the ice melts again in a day or two, the waterfowl population may not be so much different from before except for the departure of the coots and the mild-weather ducks and for the great proportions of mallards. More of the ducks have brighter plumage in late fall: the green heads of the mallard drakes stand out and so do the white throats and brown necks of the pintail drakes. The drakes of what are locally known as the "northern spoonbills" do not look like the same species as the brownish shovelers of early fall.
Mallards may be in no hurry to leave as long as they have access to cornfields and a safe place to sit between feedings. They may still be on the larger lakes by early winter.
Excerpt from OF MEN AND MARSHES by Paul L. Errington