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Determinants of Breeding Distributions of Ducks

Results


Breeding Distributions of the Species

Gadwall.—This species had a primary range centered southward of the mallard's, and was most common in the central Dakotas and the southern prairie provinces (Fig. 5A). The gadwall was less common than mallards in the northern strata. As with the mallard, highest density strata were surrounded by strata with moderate density, and densities were lower farther away. Gadwalls winter mostly in Louisiana and Texas. Perhaps 90% of spring-arriving gadwalls pass through the southern portal and only 10% come through western Montana or Alberta (Bellrose 1980). Most gadwalls arriving in spring are first exposed to wetland conditions near the southern portal.


gif --Gadwall Distribution

Figure 5. (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.

Numbers of gadwalls were not as closely correlated with local pond numbers as were mallards (Table 3). Correlation coefficients were highest in the central Dakotas (Fig. 5B), and fairly high in most strata where gadwalls were common.

Gadwall densities were associated positively with total ponds in only 5 strata in the southern prairie provinces and in eastern North Dakota (Fig. 5C). Negative correlations were found in many strata in central Canada as well as in western South Dakota, suggesting that these strata receive drought-displaced gadwalls.

Gadwalls, especially successfully breeding females, have fairly strong homing tendencies (Table 4). Often, however, they do not home until their second year, the age at which many gadwalls first breed (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.).

We conclude that most successful female gadwalls return to areas used the previous year. Young gadwalls and unsuccessful breeders likely settle in response to wetland conditions in their primary range especially in the southern part, judging from the pattern of correlation coefficients between gadwall densities and wetland counts. When many ponds are dry in the prairies, more gadwalls are counted in central and northern Canada.


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