Use of Southern Boreal Lakes by Postbreeding Canvasbacks and Redheads

Abstract

Concentrations of predominantly male canvasbacks (Aythya valisineria) and redheads (A. americana) were studied on large lakes in southwestern Manitoba during summer and fall of l969 and 1970. Peak numbers of both species occurred during the staging periods of late summer and early fall. Two lakes contained extensive beds of pondweeds ( Potamogeton spp. ), especially P. pectinatus and P. richardsonzi which apparently attracted canvasbacks. Food habits data from 14 adult canvasbacks indicated that P. pectinatus tubers and Potamogeton spp. rootstalks were preferred foods. Large numbers of redheads were observed on several lakes, including two bays of Lake Winnipegosis. One bay was intensively studied and found to have an almost continuous bed of Chara sp. and more restricted distributions of Ruppia occidentalis and P. pecttnatus. Redheads were widely distributed throughout the bay although certain areas with relatively high occurrences of R. occidentalis and P. pectinatus were used most consistently. Food contents of 1S adult male redheads were comprised predominantly of vegetative parts of these three submergents. Human disturbance of summer-molting pochards appears to adversely influence their selection of molting lakes. J. WILDL. MANAGE. 37(2): Z60-170 Although a wealth of information exists on the breeding biology of North American pochards, little information is available on their habits and requirements during the postbreeding period. Hochbaum (1959:116) observed ffie molt-migration of canvasbacks and redheads from breeding areas to large lakes in Manitoba, and Weller (1964:96) reported similar redhead movements on the basis of bag checks and limited band recovery data. The factors influencing their selection of these lakes, however, are not well known. Oring (1964) studied the behavior and ecology of postbreeding ducks in Idaho but causal factors were not considered. In view of this void in our knowledge of waterfowl ecology, this study was initiated to ( 1 ) describe physical and biological characteristics of lakes used by postbreeding canvasbacks and redheads, ( 2 ) determine the composition and dynamics of diving duck populations on these lakes, and (3) relate canvasback and redhead distribution and food habits to food resources of their summer and fall concentration areas. The study was conducted during summer and fall of 1969 and 1970. This investigation was financed mainly by a James Ford Bell Delta Waterfowl Fellowship awarded to the author through The Bell Museum of Natural History, University of Minnesota. Funding for aerial surveys was provided by the Manitoba Department of Mines, Resources and Environmental Management, Division of Research and Planning. Additional support was received from the Delta Waterfowl Research Station at Delta, Manitoba, Ionva State Conservation Commission, and Iowa State University. Special thanks are due M. W. Weller for his advice during the study and preparation of the manuscript, and to H. A. Hochbaun] for his experienced guidance during the course of the project. I gratefully acknowledge the assistance of J. R. Faaborg and D. J. Whyte in the field, and the help of R. Titman and B. D. J. Batt during aerial sur-