Diet, nutrition, and foraging strategies of breeding female canvasback (Aythya valisineria) and redhead ducks (A. americana) were studied by collecting foraging birds in 1980 and 1981 at Ruby Lake National Wildlife Refuge (Ruby Lake NWR). Female canvasbacks consumed a small number of food items, most of which were not abundant in the marsh. The diet of redheads changed with each stage of the reproductive cycle; they usually foraged on the most abundant foods in the marsh. Nutrient and energy content of the diet of canvasbacks varied little from laying through brooding, but both were quite variable in the diet of female redheads. Female canvasbacks retained large lipid reserves during formation of the clutch but used 68% from late laying to the incubation stage. Redheads expended 46% of their lipid reserves during formation of the clutch and 30% from late laying to incubation. J. WILDL. MANAGE. 49(1):203-211 Food habits and changes in body reserves of breeding waterfowl have received much attention, especially for dabbling ducks (Anas spp.) (Krapu 1974, 1981; Swanson et al. 1974b; Serie and Swanson 1976), wood ducks (Aix sponsa) (Drobney and Fredrickson 1979), and geese (Chen, Branta) (Harwood 1977; Ankney and MacInnes 1978; Raveling 1979). Generally, lipid reserves are accumulated before laying as a result of consumption of plant foods high in carbohydrates. During laying, females feed on foods high in protein (invertebrates by ducks, sprouting grasses by geese) to supply the protein necessary for egg production, and begin to utilize their lipid reserves. Restriction of food intake during incubation results in rapid depletion of lipid reserves. By the end of the incubation period, females have nearly exhausted their lipid reserves. The diet and nutrient cycles of breeding diving ducks (Aythya spp.), which feed to a greater extent on animal foods throughout the year than do dabbling ducks (Cottam 1939, Martin et al. 1951), have not been reported. W investigated the relationship between diet and body reserves of two closely related species of diving ducks, the redhead and the canvasback. These two species occur together over much of their range and have similar habitat r quirements during the breeding season (Weller 1959). However, they pursue different nesting strategies: the redhead is semiparasitic, whereas the canvasback is not (Weller 1959). Thus, their foods and nutrient cycles during the br eding season might be expected to be different, even when both species occur in the same marsh. The objectives of this study were to: (1) determine the foods consumed, preference for foods, and nutrient content of the diet of female canvasbacks and redheads; (2) determine changes in body weight and body reserves durIPresent address: 6132 SW 46th Ave., Portland, OR 97221. This content downloaded from 157.55.39.120 on Mon, 05 Sep 2016 06:18:07 UTC All use subject to http://about.jstor.org/terms 204 REDHEAD AND CANVASBACK DIETS * Noyes and Jarvis J. Wildl. Manage. 49(1):1985 ing reproduction; and (3) relate diet and changes in body reserves to their respective nesting