Diet and Nutrition of Male Canvasbacks during Postreproductive Molts

Abstract

Male canvasbacks (Aythya valisineria) have increased nutritional requirements for prebasic, down, and prealternate molts following reproduction. Because plumage and other epidermal keratins are principally composed of amino acids, molting birds may need to increase protein acquisition through selective foraging. This study was conducted to determine diet composition of postbreeding male canvasbacks relative to their stage of molt and, to investigate whether food choice was affected by protein requirements for plumage growth. The proportions of the diet composed of plant and animal foods did not change significantly (P > 0.05) during the postreproductive molts and plant material never represented <93% (aggregate dry mass) of the foods consumed by molting canvasbacks. The subterranean rootstalks and tubers of sago pondweed (Potamogeton pectinatus) were the most important food items in all molt periods except during remigial molt when birds frequently fed on emerging sago pondweed seedheads. Invertebrate foods made up only a small component of the diet in molting birds, even though these foods represented a more concentrated source of exogenous protein. Plumage growth did not alter the primarily herbivorous diet of postbreeding male canvasbacks even though they principally met protein and other nutrient requirements for molt from their diet. Changes in diet composition were related to trends in food availability rather than specific nutrient requirements for molt. Reliance on exogenous nutrients during plumage growth further indicates the need for delineation and protection of wetlands used by molting canvasbacks in western Canada and Alaska.