Evidence for some level of opportunistic settling emerged for all 10 species of ducks examined, in that their numbers in particular breeding areas fluctuated from year to year in relation to annual pond numbers. The most obvious opportunists, showing the biggest distributional changes from year to year, inhabited shallow and ephemeral wetlands (that dried out during droughts), notably Northern Pintail Anas acuta, Blue-winged Teal A. discors, Mallard A. platyrhynchos and Northern Shoveller A. clypeata. At the other extreme, showing the biggest distributional stability from year to year, were the diving ducks, such as Redhead Aythya americana, Canvasback A. valisineria and Lesser Scaup A. affinis. Their numbers showed generally poorer correlations with local pond numbers than the dabbling species, because they occupied only the deepest (most permanent) ponds and ignored the shallow temporary ones.
The distributional patterns that emerged for some species year after year were consistent with breeding habitat being filled as it was encountered on spring migration. Four species spent the winter mainly in the southern States and in spring migrated roughly south-north. They included the Gadwall Anas strepera and Green-winged Teal A. crecca (dabble-feeders), and the Canvasback A. valisineria and Lesser Scaup A. affinis (dive-feeders). In all these species, the best correlations between bird numbers and pond numbers were found in the southern parts of the breeding range, and the poorest correlations in the northern parts, supposedly reached mainly by birds unable to find accommodation further south. Three other species wintered in large numbers in the southwestern States and in spring migrated mainly north and northeast, namely the American Wigeon A. americana, Northern Shoveller A. clypeata and Northern Pintail A. acuta. The numbers of these species showed high correlations with pond numbers in southwestern count areas, but the correlations again decreased northwards. It seemed, therefore, that large segments of the populations of these seven species could have responded to wetland conditions as they encountered them on return from their wintering areas, moving progressively further in the same direction as habitats became filled. Three other species (Mallard A. platyrhynchos, Blue-winged Teal A. discors and Redhead A. americana) appeared to respond more directly to conditions in the central parts of their breeding ranges, as this was where the year-to-year correlations between population numbers and pond numbers were highest. Hence, the habitat which these three species filled first was not that which lay closest to their wintering areas.
Flexible settling, indicated by over-flight of the usual breeding areas in dry years, was shown to some extent by all species. It was most marked in dabbling ducks, notably Northern Pintail, Mallard, Gadwall, Blue-winged Teal and Green-winged Teal, but was also evident in the diving Redhead and Canvasback. All these species were displaced to the north and northwest in prairie drought years, occurring then in much larger numbers than usual in boreal and tundra regions. The American Wigeon, by contrast, was displaced to the east and northeast during drought years. These results confirmed earlier findings that, in years of drought on the prairies, more Pintails than usual overflew the prairie breeding areas to settle further north in the boreal forest and tundra of Canada, Alaska and eastern Siberia, a movement confirmed both by counts and ring recoveries (Figure 19.7; Smith 1970, Henny 1973). The magnitude of Pintail migration into eastern Asia, for example, thus depended on water conditions some 4500 km away. Breeding success was evidently poorer in the north, as shown from the ratio of young to adults shot in the subsequent winter. This rate was lowest following springs when
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