Sex-specific differences in winter distribution patterns of canvasbacks

-Winter band recovery distributions of North American Canvasbacks (Aythya valisineria) suggested that males and females exhibit comparable degrees of fidelity to general wintering areas. Of birds banded during the winter, the proportion of males was found to be higher in northern than in southern areas. Winter band recovery distributions of birds banded in particular areas during the summer were found to differ significantly between sexes, with females being recovered farther south. Factors that may have affected the evolution of sex-specific wintering distributions include: (1) possible reproductive benefits derived by males who winter in the north and thus reach northerly breeding areas early; (2) sexual dimorphism in body size, which may render the smaller females especially susceptible to periods of inclement weather and food shortages; and (3) interactions between sexes in which males may control food supply when food is scarce. Two lines of evidence from field data on Canvasbacks in the Chesapeake Bay suggest the existence of competition between males and females. First, Canvasbacks trapped during winter in smaller bodies of water tended to have higher proportions of females and weigh less than birds trapped in large open bodies of water. Second, analysis of aerial photographs of wintering rafts of Canvasbacks showed patterns of intersexual segregation, with females being found more frequently on peripheral areas of rafts. Lack (1954, 1966) and Fretwell (1972) have suggested that "limitation" of temperate bird populations most likely occurs during the winter or nonbreeding season. It is therefore of interest to examine the distribution and abundance of bird populations during this period of the year. In many species of migratory birds in the Northern Hemisphere, males appear to winter farther north than females (see Nice 1933, Lack 1944, Selander 1966, Ketterson and Nolan 1976 and references therein). It has been suggested, (Witherby et al. 1939:219) that female and young diving ducks in Great Britain generally migrate farther south than adult males. Evidence of such a sexual difference has been provided in North America for Common Mergansers (Mergus merganser; Anderson and Timkin 1972) and Buffleheads (Bucephala albeola; Erskine 1971), in Europe for Common Pochards (Aythya ferina) and Tufted Ducks (Aythya fuligula; Salomonsen 1968), Mallards (Anas platyrhynchos; Lebret 1950, Nilsson 1976), European Wigeon (Anas penelope), Greenwinged Teal (Anas crecca), Northern Shovelers (Anas clypeata), and Pintails (Anas acuta; Lebret 1950). Bellrose et al. (1961) summarized evidence suggesting that sex ratios of North American waterfowl vary geographically during the winter. Hochbaum (1944:132) suggested that Canvasbacks (Aythya valisineria) may exhibit a differential distribution of sexes on the wintering grounds. Allen (1931:354) and DeGraff et al. (1961:79-80) noted the high proportion of males among Canvasbacks wintering in New York, and suggested that females were more numerous farther south. The possibility that male and female Canvasbacks winter in different areas is interesting from both biological and management points of view. The continental Canvasback population is thought to exhibit the most disparate sex ratio (65-70% male) of all North American ducks. This ratio (and the low "reproductive potential" which it implies) has been labeled "a fundamental problem in the population dynamics of the Canvasback" (Trauger 1974). Canvasbacks hatch approximately equal numbers of males and females (Hochbaum 1944:51, Sowls 1955:164); hence, the disparate population sex ratio must be due entirely to differential mortality. Indeed, in an analysis of Canvasback banding and recovery data, we found mortality rates to be significantly greater in females than males in each of the