When plants or animals are dimorphic, they occur in two visibly distinct forms.
For example, many types of birds are dimorphous. The males and females look different enough for them to be visually sexed. In budgerigars, the birds can be sexed because males and females have ceres of different colors. A more extreme example is found in the eclectus, where the male is predominantly green and the female is mainly scarlet; furthermore, the male's beak is orange and the female's is black. The difference between them is so striking that explorers originally thought they were two entirely different types of birds.
