During 1990-95, we studied sympatrically breeding greater and lesser scaup on islands of the North Arm of Great Slave Lake near Yellowknife, Northwest Territories.
The discriminant function, based on measurements of lengths and widths of 432 lesser and 471 greater scaup eggs, was 98% accurate for classifying individual eggs (Fournier and Hines, unpubl.
The discriminant function was used to classify some clutches to species and also to identify possible instances of inter-specific nest parasitism involving both lesser and greater scaup.
We found 605 scaup nests on the islands of the North Arm: 300 nests of greater scaup, 170 of lesser scaup, and 135 for which the species was not determined.
We found greater scaup nests on 89 islands and lesser scaup nests on 55 islands.
The distribution of lesser and greater scaup nests on the North Arm is indicated in Figure 1.
Habitat characteristics of greater and lesser scaup nesting islands and of the entire sample of islands visited are presented in Table 1.
This relationship held for greater scaup (98%), lesser scaup (97%), and unidentified scaup (99%) nests.
Lesser scaup were apparently influenced by island size (larger islands had more nests), distance from shore (nearshore islands had more nests), and tree cover (islands with fewer trees had more nests) (Table 2).
Islands used by nesting greater scaup were located, on average, farther from shore than those used by lesser scaup (Table 1).
Lesser scaup nests were found in the YKSA on the peripheries of 48 different ponds, including 25 natural and 23 man-made ponds.
In contrast to island-nesting scaup, lesser scaup on the YKSA made little use of low shrubs for nesting cover (one nest in sweet gale), and they occasionally used einergents such as cattail, horsetail, marsh cinquefoil (Potentillapalustris) and water-arum (Calla palustris), species not widely available to island-nesting scaup.