In the past 30 years, canvasback ducks (Aythya valisineria) have switched from traditional midwest staging areas on inland lakes and the Illinois River to backwater pools of the Upper Mississippi River (UMR). An energetic approach that integrated field observations of free-living canvasbacks with controlled laboratory experiments on captive ducks was used to study fall staging on Lake Onalaska (Navigation Pool 7, UMR) near LaCrosse, Wisconsin. Time-activity budgets of canvasbacks differed by sex, year, date, time, and weather. Males spent more time foraging than females (20.8% to 17.9%), and canvasbacks foraged more in 1983 than in 1984 (24.4% to 15.8%), during the peak week of staging (22.4%), and at night (28.0%). The staging population of canvasbacks on Lake Onalaska averaged 83% male, but females may have stayed for a shorter time than males. Energetic costs of resting, captive ducks were found to increase (16.9 to 43.8 KJ/h) as water temperature decreased (35 to 0(DEGREES)C). Mean resting costs on water were 2.0 times a basal metabolic rate (BMR), and diving costs were 7.7 times BMR. In controlled studies of functional response, canvasbacks foraging on American wildcelery (Vallisneria americana) winter buds consumed 360/h at densities above 30/m('2). Males were more efficient foragers than females. Winter buds near the substrate surface were taken more easily than winter buds located deeper. Canvasbacks metabolized 79.2% or 2.32 KJ of a winter bud. An estimated 0.18 winter buds/dive or 2.7 clams/dive are needed to exactly balance diving costs. Staging canvasbacks assimilated 2600 KJ/day, and assuming that 50% of this energy is converted to fat, they gained 32.4 g of fat/day.