When they pair off with mating partners, often one or several drakes end up "left out".
This group sometimes targets an isolated female duck, even one of a different species, and proceeds to chase and peck at her until she weakens, at which point the males take turns copulating with the female.
Male Mallards also occasionally chase other male ducks of a different species, and even each other, in the same way. In one documented case of "homosexual necrophilia", a male Mallard copulated with another male he was chasing after the chased male died upon flying into a glass window.