Female, immature and eclipse male Mallards are much paler, without contrast between head and body and with white borders to a blue speculum. Hybrid Mallard x Black Ducks are often seen and usually have traces of the mallard plumage, with bluer speculum usually bordered with white on one or both edges. Mottled Duck is similar, with broader brown edgings to upperparts and bluer speculum. Female Gadwalls have a steeper forehead, gray bill, white belly, and usually shows white in the wing.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.