Northern Prairie Wildlife Research Center
Diets of canvasbacks breeding on Ruby Lake, Nevada, varied with stage of reproduction (Noyes and Jarvis 1985). Plant material comprised more than 90% of the diet of prelaying canvasbacks on Ruby Lake (Noyes and Jarvis 1985), which was more similar to the diet of migrating canvasbacks in North Dakota than to the diets of first arrival canvasbacks (Barzen and Korschgen, unpubl. ms.), or of prenesting or RFG birds in Minnedosa. Differences in prelaying diets between the two areas may result in part from differences in migration chronology and foraging opportunities. Unlike canvasbacks moving through the Atlantic and Mississippi flyways to southern Manitoba, canvasbacks migrating to Ruby Lake in spring have a shorter migration and few foraging opportunities between their wintering area and Ruby Lake. Also, canvasbacks at Ruby Lake have a longer interval between arrival and nesting (Noyes 1983, Austin and Serie, unpubl. data) in which to feed and build up energy reserves. Lack of information on yearly and seasonal availability of invertebrate and plant foods limits direct comparisons of food use versus availability among studies.
Canvasbacks breeding in the prairie pothole region shift from plant material during spring migration to increased proportions of animal foods during breeding. Spring migrants on the Missouri coteau and drift prairie of North Dakota consumed only plant foods, primarily pondweed tubers (Barzen and Korschgen, unpubl. ms.). Canvasbacks collected soon after arrival at Erickson, Manitoba (35 km north of Minnedosa), consumed more plant food (primarily pondweed tubers [53% of the diet]) than animal foods (38% animal material, exclusively midges) (Barzen and Korschgen, unpubl. ms.). Diets of prenesting birds at Erickson (Barzen and Korschgen, unpubl. ms.) and Minnedosa (this study) were similar. Our results indicate that the volume of animal foods in the diet did not continue to increase after arrival on the breeding grounds.
Continued importance of plant foods to female canvasbacks throughout reproduction contrast with the mostly invertebrate diets of other prairie-breeding ducks, and does not fit current theories of nutritional ecology for breeding anatids, i.e., that female ducks meet the protein requirements of reproduction by consuming a high proportion of animal (proteinaceous) foods (Krapu and Reinecke, in press). Lack of a large increase in animal foods in the diet and lack of changes in protein reserves during reproduction (Barzen and Serie 1990) suggest that canvasback females were able to obtain sufficient protein from their diet of pondweed tubers and invertebrates. Pondweed tubers are high in metabolizable energy and, relative to other plant foods, high in protein and low in fiber (Sugden 1973, Anderson and Low 1976). Consumption of tubers by females also may serve to offset energy costs of incubation or renesting. Because of the nutritional value of pondweeds and their importance to canvasbacks during breeding and migration, management of breeding canvasbacks must consider production and distribution of pondweed tubers as well as that of invertebrates.