This was essentially a prairie duck and very rare in Ontario until quite recently: in my early birding days it was always a red letter day when a Gadwall was spotted among the more common puddle ducks. Nowadays it is not unusual to find 100 in favourable localities such as Lynde Creek marsh in autumn or the New Toronto waterfront in winter.
Males in breeding plumage are gray with black rear ends. Females look like somewhat smaller female Mallards, but like the males with a spot of white where the Mallard has its blue speculum bordered with white along the rear edge of the wing. Sometimes this white spot does not show when they are at rest on the water, but when it does show it is diagnostic. The gray bill bordered with orange-yellow along the sides is also characteristic.