Sex and Age Ratios and Weights of Common Mergansers

Abstract

Age and sex ratios and weights of common mergansers (Mergus merganser) were obtained between the fall 1966 and the spring 1972 from southeastern South Dakota, southwestern Minnesota and the Great Salt Plains National Wildlife Refuge in northwestern Oklahoma. A total of 319 specimens were weighed, and sex and age ratios were obtained for more than 12,000 mergansers. Immature mergansers were most abundant in late November in Minnesota and South Dakota and in December and early January in Oklahoma. As winter temperatures became colder, the average weights of mergansers and the proportion of adult males increased in both regions. In addition, both the weight increase and the proportion of adult males in the population were greater in South Dakota than in Oklahoma. Seasonal and geographic variation in sex and age ratios and in weight probably reflect adaptations to varying climatic conditions. Adult males, with their larger bodies, are probably more cold-tolerant than members of the other ageand sex-classes. COMMON MERGANSERS * Anderson and Timken 1127 Harcourt, Brace and Company, New York and Burlingame. 440pp. STOUr, I. J. 1967. The nature and pattern of non-hunting mortality in fledged North American waterfowl. M.S. Thesis. Virginia Polytechnic Inst., Blacksburg. 31Spp. THOMFORDE, L. L. 1970-1971. Survival of hand-reared mallards ( Anas platyrhynchos ) on altificial farm ponds. J. Minnesota Acad. Sci. 37 ( 1 ) :23-26. TINBERGEN, N., M. IMPEKOVEN, AND D. FRANCK. 1967. An experiment on spacing-out as a defence against predation. Behaviour 28(pt. 34 ) :307-321. ( German surnmary. ) WELLS, R. A. 1951-1952. Dividends in ducks. Part II. The program for small marshes. New York State Conservationist 6(3):25-26. Received for publication April 4, 1970. Between the fall of 1966 and February 1972 we studied food habits (Timken and Anderson 1969), sex and age characteristics ( Anderson and Timken 1971 ), weights, and sex and age ratios of common mergansers in South Dakota, Minnesota, and Oklahoma. Our purpose in the present report is to consider geographic and seasonal differences in age and sex ratios and in weights. Common mergansers are abundant on rivers and lakes in the study region in late fall and early spring. At least a few individuals are likely to spend the winter anywhere open water and food are available. Common mergansers are rare or absent as breeding birds over all but the northern portions of the central United States (AOU 1957) . We thank R. R. Cody, T. Fuller, and L. Nelsen for their aid in collecting and salvaging mergansers in South Dakota and K. Beaty, W. Naberhaus, M. Reeder, and R. Wilson for their assistance in Oklahoma. We are indebted to the Department of Biology at the University of South Dakota and to the National Aeronautics and Space Administration for financial and logistic support. Preparation of this report has benefited from discussions with D. G. Dunlap, B. E. Harrell, P. F. Nighswonger, and