Carcass Composition of Nonbreeding Blue-Winged Teal and Northern Pintails in Yucatan, Mexico

We studied the carcass composition of Blue-winged Teal (Anas discors) and Northern Pintails (A. acuta) wintering in Yucatan, Mexico from October through March 1986-1987 and 1987-1988. Patterns of body mass and lipid reserves in Yucatan were similar to patterns reported in ducks (including conspecifics) wintering farther north. However, on average, ducks in Yucatan weighed 5-22% less and carried smaller energy reserves. We argue that an endogenous mechanism is the primary factor controlling the cyclic pattern of body mass and carcass composition of ducks during winter, but that environmental conditions are the proximate cause responsible for the observed geographical variation in the amplitude of changes.