Nutritional implications of molt in male Canvasbacks: Variation in nutrient reserves and digestive tract morphology

Body composition and gut morphology of molting male Canvasbacks (Aythya valisineria) were investigated in central Alberta from termination of breeding activities until fall migration (May-October) in 1989-1990. During this postreproductive period, male Canvasbacks underwent prebasic, down, and partial prealternate molts. Protein, lipid, and mineral dynamics were analyzed to determine if male Canvasbacks relied on endogenous nutrient reserves to meet increased nutritional demands during molt. In addition, mass and length of digestive organs were analyzed to document changes in the digestive tract that could provide endogenous protein (i.e., via muscle tissue catabolism) and/or possibly enhance exogenous nutrient assimilation for molt. Ingesta-free body mass of male Canvasbacks reached its annual, minima during remigial molt ((x) over bar = 973.5 g) and fluctuated primarily in response to storage and catabolism of lipids. Body composition data indicated little reliance on endogenous fat, protein, and mineral to satisfy nutrient requirements for molt. Lipid reserves were not accumulated prior to molt, but instead were metabolized from arrival on molting areas through mid-remigial molt. Premigratory lipogenesis started during mid-remigial growth and occurred simultaneous to elevated molt intensity, indicating that energetic requirements for molt were met exogenously. Despite the relative stability of total body protein during postreproductive molts, there were marked shifts in the distribution of muscle tissue in molting birds. Pectoral muscles atrophied from arrival on molting habitat through mid-remigial molt, concurrent with hypertrophy of leg and some digestive tract tissues. Increased mass of the digestive tract during remigial molt originated principally from growth of the gizzard. The digestive tract returned to its original mass as birds regained the ability to fly and their diet reverted back to foods with lower fiber content. Because postreproductive molts were extended over six months, male Canvasbacks had relatively low daily nutritional demands that could be met by dietary intake.