. The Canadian field-naturalist. Wm iWrl. Island C - zones 2 herbicide applied T 1 June July July Aug. June July July Aug. June July July Aug. June July July Aug. 1966 Figure 4. Phenological increase in vegetative cover in the zones on Islands A and C that were treated once with herbicide in 1966 but not in 1967. Relationship between Nesting Waterfowl and Vegetation Newly established duck nests were recorded over a period of 2 months, from early May to early July; consequently, they were associated with both old and new growth. Early-nesting species (Mallard, Pintail, and American Wid- geon) u
. The Canadian field-naturalist. Wm iWrl. Island C - zones 2 herbicide applied T 1 June July July Aug. June July July Aug. June July July Aug. June July July Aug. 1966 Figure 4. Phenological increase in vegetative cover in the zones on Islands A and C that were treated once with herbicide in 1966 but not in 1967. Relationship between Nesting Waterfowl and Vegetation Newly established duck nests were recorded over a period of 2 months, from early May to early July; consequently, they were associated with both old and new growth. Early-nesting species (Mallard, Pintail, and American Wid- geon) usually nested among dead vegetation from the previous year, and late-nesting species (Lesser Scaup, Gadwall, and White-winged Scoter) nested mainly among new growth. Of all duck nests recorded on Islands A and C during three seasons of this study, between 28 and 30% were initiated in May. Mallard, Pin- tail, and American Widgeon were responsible for 80 to 100% of these early nesting attempts. Before we could evaluate changes in the availability of the various types of residual cover, we had to determine the extent to which ducks exhibited preferences for the various types of vegetation. We did this by recording the number of nests associated with each of the dominant plants and compared these data with the availability of the respective plants, in terms of area dominated by them on each island (Table 2). Apart from differences in vegeta- tion, both islands were relatively homogeneous in all other characteristics. Thus the difference between the percentage of the island under a dominant plant and the percentage of duck nests found therein was interpreted as evidence of preference or avoidance on the part of the waterfowl. Differences between utilization and availability of the various plants considered were significant for grasses, bulrushes, and mis- cellaneous plants. Residual growth of grasses and plants in the miscellaneous category were avoided by nesting Mallards, Pintails
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