A study was conducted in Larimer County, Colorado, 1967-70, to investigate the ability of waterfowl to winter north of the 0 degree C isotherm. Ground and aerial counts were made of selected duck and goose concentration areas where waterfowl maintained patches of open water. The number of birds using an area and the size of the area were estimated from aerial photographs. The area of open water per bird and ambient temperature appear to be directly related. Ten 3-hour experiments were conducted with captive, wild Mallards in a controlled temperature, cold room to determine the effect of ducks on low water temperatures. Ducks under test condtions produced significantly more heat than ducks under control conditions. The test condition metabolism represented an increase of 22.7 percent over the control metabolism. Nocturnal behavior of Redhead ducks on a pond was observed during freezing weather. Activity was greater at night during freezing weather than during warmer weather. Duck defecation rates and chemical content of feces were determined from captive ducks. (WIRSIC abstract)