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Breeding Range. (Fig. 28). Fairly common in the Turtle Mountains; fairly common locally throughout the Missouri Coteau and in portions of the Northeastern Drift Plain (most numerous in southwestern Cavalier County and in the northern half of Nelson County); uncommon on the Northwestern and Southern Drift Plains; rare in the Agassiz Lake Plain and Southwestern Slope Regions.

Breeding Habitat. Optimum habitats are represented by cover types 2 and 3, slightly brackish and moderately brackish, semipermanent ponds and lakes with emergent cover dominated by hardstem bulrush, cattails, or a mixture of the two. Occasionally, river bulrush is the dominant emergent of ponds that are occupied. Canvasbacks also occur more sparingly on brackish and subsaline semipermanent ponds and lakes in which alkali bulrush is an important cover species and in seasonal ponds that are dominated by whitetop or a mixture of whitetop and slough sedge. Locally, considerable use is made of shallow river impoundments that are managed for waterfowl.

Nesting. Breeding season: Late April to early September; peak, mid-May to mid-August. Extreme egg dates (50 nests): April 28 [1963] to July 15 [1964] in Stutsman County (H. A. Kantrud, RES). Of all nests with viable eggs that were found (excluding a single nest in April), 45 percent were recorded in May, 48 percent in June, and 7 percent in early July. Extreme dates of dependent young (410 broods): May 29 [1963] in Stutsman County (RES) to September 5 [1969] in Nelson County (P. A. Aus). Of all broods that were recorded (excluding a single brood seen in late May), 8 percent were observed in June, 57 percent in July, 34 percent in August, and 1 percent in early September.

Nests were ordinarily situated over water in fairly dense stands of emergent vegetation. Water depths measured at 34 nest sites ranged from 3 1/2: to 30 inches, with a mean of 17 inches. The prevalent nesting cover used by Canvasbacks was composed of hardstem bulrush, cattails, or both species in combination. They were found to be the principal cover species at 35 nest sites (76 percent) out of 46 nests that were studied intensively. Locally, alkali bulrush, river bulrush, slender bulrush, and whitetop were fairly important as cover species.

Indicated clutch size (26 nests): 6 to 16 eggs; mean, 9.9 eggs. In addition, a few obvious dump nests containing from 18 to 21 eggs were found.

Figure 28. Breeding Range of Canvasback.

Canvasbacks