Mallard

Note in the first pic how the bird has the head of a male, but the body of a female. This bird is in the middle of the process of changing his feather coat. This takes place after mating-season (he´s changing it from the feather coat of a male to a female) when the males do not have to impress the females anymore. It is also done in order to reduce agression from other males which will lessen the stress and extend their lives.