Gadwall, Fernhill Wetlands, Forest Grove, Oregon on 20 February 2006 by Greg Gillson.
Historically, Gadwalls wintered on open water in the Pacific NW, primarily west of the Cascades to the coast, and bred east of the Cascades in the larger tule lakes there.
Numbers of breeding birds west of the Cascades in Washington and Oregon have been quite low and infrequent. That is, until quite recently. In the Washington state breeding analysis the habitat preference for western Washington is described as "urban wetlands." Likewise, Gadwalls began breeding west of Portland, Oregon about 2002, and are now widespread, but not numerous, breeders throughout wetlands there, including Fernhill Wetlands in Forest Grove, Jackson Bottom Wetlands in Hillsboro, and Tualatin River NWR at Sherwood, to name only a few breeding sites.
This increase in abundance is part of a continent-wide increase in numbers and expansion in range since the 1950's. Another peak in numbers occured in the 1990's.
Gadwall are a bit smaller than Mallard, and the females look quite similar at first glance. Male Gadwall may look like female of some other ducks, but the black bill and black rear end are quite distinctive, as is the small white wing patch in both sexes.
Though not colorful, the drake Gadwall at close range is quite handsome. Though looking gray at a distance, the breast and sides are vermiculated with fine black and white patterned lines. The scapulars are rusty-colored.
Gadwalls nest later than Mallards. While Mallard chicks may be seen most commonly in May and June, Gadwall chicks are noted with hens most frequently during July and August.
After nesting, adult ducks molt all flight feathers at once and are flightless 6-8 weeks. You may note the ponds are covered with duck feathers in summer. The drakes undergo a molt into an "eclipse" plumage where they look similar to the females, but can be identified with close observation. I suggest spending a bit more time this summer looking at those duck families in the pond. You just may find that some are Gadwall, and not Mallard as you presumed.