Effects of diet on rate of body mass gain by wintering canvasbacks

Because habitat degradation has led to the loss of submerged vegetation in Chesapeake Bay, wintering canvasbacks (Aythya valisineria) have shifted from a plant diet of American wildcelery (Vallisneria americana) to an animal diet of Baltic clams (Macoma balthica). We conducted experiments with pen-reared canvasbacks (n = 32, 1990; n = 32, 1991) to assess the effect of this diet change on mass recovery rate following a simulated period of food deprivation. During the recovery phase, canvasbacks were fed ad libitum either (1) Baltic clams (1991 only), (2) tubers of wildcelery, 3) corn, or (4) commercial control diet. Initial body mass of ducks did not differ between years (P = 0.754) or among pens (P > 0.264) or diets within years (1990, P = 0.520; 1991, P = 0.684). Body mass decline during food deprivation (x̄ = 26.0 g/day ± 0.6 SE) did not differ among diets (1990, P = 0.239; 1991, P = 0.062) or between sexes in 1990 (P = 0.197), but was greater (P = 0.039) for males (x̄ = 28 g/day ± 0.8 SE) than females (x̄ = 25 g/day ±  0.9) in 1991. Mass recovery rate differed between diets (clams excluded) in 1990 (P = 0.003) and 1991 (clams included) (P = 0.011); mean = 42 g•bird^-1/day^-1 ± 3.8 (SE) control diet, mean = 32 g•bird^-1/day^-1 ± 2.8 wildcelery tubers, mean = 24 g•bird^-1/day^-1 ± 4.9 whole corn, and mean = 23 g•bird^-1/day^-1 ± 1.0 Baltic clams. Canvasbacks consumed an average of 2,169 g•bird^-1/day^-1 of Baltic clams, 1,158 g•bird^-1/day-¹ of wildcelery tubers, 152 g•bird^-1/day^-1 whole corn, and 208 g•bird^-1/day^-1 (dry mass) control diet during recovery. Managers should restore and maintain aquatic plant foods that enhance winter survival of canvasbacks and other waterfowl in response to declining habitat quality.