Changes in the visceral morphology of wintering female mallards (Anas platyrhynchos)

Gizzard, heart, intestine, kidney, liver, and spleen mass and cecal length were measured from 143 adult female mallards (Anas platyrhynchos) collected during the winters of 1981 – 1983 in the Mingo Basin of southeastern Missouri. All visceral organs of female mallards changed over winter and seemed responsive, and adaptive, to changes in nutritional and physiological demands attendant with changing annual cycle events. Intestine and gizzard mass and cecal length were greatest during the prealternate moult in early fall and before spring migration, both periods of high total food and fibre intake. Kidney mass was also highest during periods when females foraged intensively and may have reflected the need to excrete large quantities of metabolic by-products. Liver mass increased at the initiation of the prebasic moult and immediately before spring migration, probably in response to increased intermediary metabolism of both proteins and lipids. Spleen mass increased during moult periods and probably facilitated increased blood production and circulation associated with feather development. Heart mass was low in fall and high during the prebasic moult and before spring migration.