"In mid-summer after breeding, the male Mallard has a complete molt, producing a dull-colored basic plumage, aptly termed the eclipse plumage. Male Mallards in eclipse plumage look remarkably like females, but their bills are light olive green, while females' are orange marked with black. Because flight feathers are also molted at this time, the birds become temporarily flightless and tend to be very secretive. The male Mallard's basic plumage is kept only a few weeks; it is soon lost in a molt of the body feathers which produces the brightly colored head and other distinctive features of the breeding plumage. The timing of this molt is related to courtship in Mallards, which begins in the fall." ~ The Cornell Lab of Ornithology
Male Mallard in eclipse plumage
Female Mallard