Gadwall

The Gadwall is a medium-sized dabbling duck whose subtle pattern rewards close inspection. Breeding males are finely vermiculated grey with a black rear, chestnut wing coverts and a white speculum. Females are mottled brown and Mallard-like, but show a slimmer dark bill edged orange and the same white wing patch. Males in eclipse plumage become browner while retaining darker rear quarters (Wikipedia, general species account).

Gadwalls inhabit marshes, lakes, reservoirs and sheltered coastal water. They feed mainly on aquatic vegetation and seeds by dabbling or upending, while invertebrates become more important during breeding. They sometimes steal plants brought to the surface by diving birds. The female nests in dense ground vegetation and tends the ducklings alone. Migratory populations form flocks on temperate wintering wetlands, including many sites in Japan (Wikipedia, general species account).