DETERMINANTS OF BREEDING DISTRIBUTIONS OF DUCKS

The settling of breeding habitat by migratory waterfowl is a topic of both theoretical and practical interest. We use the results of surveys conducted annually during 1955-81 in major breeding areas to examine the factors that affect the distributions of 10 common North American duck species. Three patterns of settling are described: homing, opportunistic, and flexible. Homing is generally more pronounced among species that use more stable (more predictable) wetlands, such as the redhead (Aythya americana), canvasback (A. valisineria), lesser scaup (A. affinis), mallard (Anas platyrhynchos), gadwall (Anas strepera), and northern shoveler (Anas clypeata). Opportunistic settling is more prevalent among species that use less stable (less predictable) wetlands, such as northern pintail (Anas acuta) and blue-winged teal (Anas discors). Flexible settling is exhibited to various degrees by most species. The 10 species are shown to fall along a natural ordination reXecting diSCerent life history characteristics. Average values of indices of rand K-selection indicated that pintail, mallard, blue-winged teal, and shoveler have the most features associated with unstable or unpredictable environments. Gadwall, American wigeon (Anas americana), and green-winged teal (Anas crecca) were intermediate, and attributes of the diving ducks were associated with the use of stable or predictable nvironments. Some species notably mallard, gadwall, blue-winged teal, redhead, and canvasback tend to fill available breeding habitat first in the central portions of their range, and secondly in peripheral areas. Other speciesAmerican wigeon, green-winged teal, northern shoveler, northern pintail, and lesser scaup fill their habitat in the order it is encountered uring spring migration. Age and sex classes within species vary in their settling pattern. Some of this variation can be predicted from the mating systems of ducks in which breeding females, especially successful ones, have a greater investment in habitat resources and are more likely to return to the same area in subsequent years. WILDL. MONOGR. 100, 1-37 6 WILDLIFE MONOGRAPHS