Mallard (Anas platyrhynchos) Science Article 4
It is a peculiarity of most species of ducks that in the early summer, shortly after the breeding season, the males moult and then assume a plumage which in coloration and form closely resembles that of the female. This is known as the ‘eclipse’ or ‘summer ‘ plumage. It lasts some weeks until a second moult, when the ‘normal’ or ‘mating’ plumage is again restored. During eclipse the flight feathers are replaced and the male usually deserts the female with her brood and retires either alone or with other eclipsing drakes into seclusion. The plumage changes during eclipse in various species are exceedingly intricate, and in many of the rarer species are incompletely known. Witherby (1924) publishes minute descriptions of the plumage changes of most of the British species and his descriptions are largely relied upon for the following account.
Arthur Walton, J. Exp. Biol., Oct 1937;14:440-447