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Wildlife Monographs

Determinants of Breeding Distributions of Ducks


Settling Patterns

We found evidence that ducks exhibit all 3 of the hypothesized patterns of settling. Our literature review suggested that, within species, homing was more prevalent among adult females than among males (for the few species in which males have been studied) or yearlings. That adult females are more philopatric than adult males is consistent with the seasonal monogamy of these species and the greater role of the female in parental care (Rohwer and Anderson 1988); male dispersal may be largely a consequence of when and where the ducks form pair bonds. Because of differences in parental care, the reproductive success of males is limited by access to fertilizable females, whereas the reproductive success of females is limited by environmental resources needed for reproduction. Because females spend far more time than males at nests and with young, they are exposed to greater predation risk (Sargeant 1972, Sargeant et al. 1984), which subsequently results in a preponderance of males in the population (Johnson and Sargeant 1977). Thus, all females can generally find a mate, and selection should favor those that are best able to find quality breeding habitat, which includes secure nest sites and access to food during egg laying and incubation. Females may tend to home because familiarity with a breeding area should facilitate access to both food and a secure nest site. For males not all of which can mate, the selective pressure to obtain a female confers an advantage to pairing as early as possible, on the wintering grounds or during spring migration. Because habitat needs during the breeding season are less critical for males than for females, there is relatively little advantage to males in homing to familiar sites. Instead, the male follows the homing female (McKinney 1965). Greater philopatry by females also is generally consistent with the prediction of Greenwood's (1980) hypothesis and the social structure of duck populations. The female does not select a mate on the basis of the quality of his territory. Accordingly, Greenwood's (1980) hypothesis predicts that females are more likely to home than males.

For the mallard, gadwall, and northern shoveler, at least, there is evidence that hens that are successful in hatching a clutch are more prone to home than those that are not (Doty and Lee 1974; Mihelsons et al. 1986; J. T. Lokemoen, H. F. Duebbert, and D. E. Sharp, Reproductive strategies of prairie mallards, gadwalls, and blue-winged teal, unpubl. manuscript, U.S. Fish and Wildl. Serv., Jamestown, N.D.). This phenomenon could be viewed as a "play-the-winner" approach to settling in a habitat; there is no advantage to be gained by changing a habitat that has been successful. In fact, some studies have shown increased reproductive success among birds that used nest sites in which they were previously successful (Dow and Fredga 1983, Blancher and Robertson 1985).

Contrasting the 10 species, homing seems most prevalent among redhead, canvasback, lesser scaup, mallard, gadwall, American wigeon, and shoveler. These species generally use more stable wetlands than the others (Smith 1971, Stoudt 1971, Stewart and Kantrud 1973); their use of these more stable habitats would predict a greater homing tendency (e.g., Wiens 1976).

All species exhibited opportunistic settling to some extent, in that their numbers fluctuated with pond numbers. Such movement is especially important in spatially heterogeneous habitats (Gauthreaux 1982), such as wetlands in the prairie, where drought is often a local phenomenon. Most notable as opportunists were pintail, blue-winged teal, mallard, and shoveler. Opportunism is favored when habitat conditions are unpredictable from 1 year to the next. The species using the least predictable wetland habitats is the pintail, which also is the species most strongly correlated with pond counts. Diving ducks, at the other extreme, use more predictable wetlands; correspondingly, these species were less closely tied to pond counts. More conclusive evidence for opportunism is a pattern of strongest correlations with pond counts in strata first encountered by returning birds in the spring. This pattern was apparent for American wigeon, green-winged teal, and canvasback and was pronounced for gadwall, shoveler, and pintail.

Flexible settling, indicated by overflight in dry years, was shown by most species when appropriate. Drought displacement was generally to the north or northwest, as suggested earlier by Hansen and McKnight (1964), Crissey (1969), and Smith (1970). Some overflight to the northeast was indicated for mallard, gadwall, blue-winged teal, green-winged teal, redhead, and canvasback. For American wigeon, many of which enter from the west, drought displacement to the east and southeast was seen.

Filling the Habitat

Nudds (1978,1983) suggested that the habitat of ducks becomes filled first closest to wintering grounds, which is consistent with predictions of Fretwell and Lucas (1969). Nudds cited unpublished work of H. A. Kantrud, who found that correlations between pond density and density of breeding-duck pairs decreased with increasing latitude. That result, however, was based on counts of all species combined. In contrast, the 10 species examined in this report filled the habitat somewhat differently. Four of them—gadwall, green-winged teal, canvasback, and lesser scaup—did display a latitudinal decline in correlation with pond numbers, although none of these species was closely associated with pond counts. The 3 species that winter in large numbers in the western states— American wigeon, northern shoveler, and northern pintail—showed high correlations with pond numbers in southwestern strata, and the latter 2 did in several southern strata as well. Although it is possible that many birds actually home but then backtrack to areas encountered earlier in migration, it seems clear that large segments of the populations of these 7 species respond to pond conditions as they encounter them.

The remaining species—mallard, blue-winged teal, and redhead—appear to respond more directly to wetland conditions in the primary portion of their breeding ranges, as is evident from highest correlations with pond numbers there. These findings fit the ideal-free distribution model of Fretwell and Lucas (1969), who predicted that the "most suitable" habitat fills first and that some individuals are forced to less suitable habitats. For several species of ducks, the most suitable habitat may not be that closest to wintering areas, as Nudds (1978) conjectured, but rather that habitat in the primary part of the breeding range.

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