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Determinants of Breeding Distributions of Ducks

Results


Breeding Distributions of the Species

Lesser Scaup.—The distribution of scaup was very different from the other species, with highest densities attained in the north and in 2 strata mostly in Manitoba (Fig. 13A). Scaup were uncommon south of Canada. Although aerial observers cannot distinguish lesser scaup from greater scaup (Aythya marila), it appears that greater scaup predominate in coastal strata of Alaska and the Northwest Territories (strata 1, 8, 9, 10, 11, and 13), whereas most of the scaup (an estimated 82%) in the interior strata are lesser scaup (Bellrose 198O, Boyd 1983). Nonbreeding lesser scaup may summer far south of their customary breeding areas (Wetmore 1920); therefore, the distribution suggested by the aerial surveys may not be the true breeding distribution. Also, lesser scaup migrate late in spring and may be counted in strata south of their ultimate destination (A. D. Afton, pers. common.).


gif --Lesser Scaup Distribution

Figure 13. (A) Average species density by stratum, 1955-81; (B) correlation between species density and local pond counts, 1955-81; and (C) correlation between species density and total pond counts, 1955-81.

Lesser scaup winter mostly along the Atlantic and Gulf coasts (Bellrose 1980). The fraction of them returning through the southern portal to the breeding grounds probably exceeds 90%. Greater scaup winter along the coasts of both oceans, and sparingly in the Gulf of Mexico and the Great Lakes (Bellrose 1980). Their primary migration pathways are either along the Pacific Coast or northwestward across the Great Lakes.

Correlations between scaup densities and counts of local ponds were low (Table 3). The highest values were calculated for 9 strata in the southern part of the surveyed area, none of which had large numbers of scaup (Fig. 13B). This pattern of correlations may reflect the presence of nonbreeding scaup in the southern part of the area, alluded to above.

Lesser scaup tended to correlate positively with total ponds in many strata in central and southern Canada and the United States (Fig. 13C), including many strata in which the species was not common. Negative correlations were found only in 3 northern strata and 1 in Manitoba, strata that contained large numbers of scaup in most years (Fig. 13A).

Lesser scaup are strongly philopatric (Table 4) and only somewhat affected by local wetland conditions. Nonetheless, the strata showing highest correlations with pond counts are south of the primary range of the species (Fig. 13A), indicating that some birds, perhaps nonbreeders, fill the habitat as they encounter it. Displacement appears to be to the northwest and possibly to the east.


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