Cockatiel : Sexual dimorphism

All wild cockatiel (also known as the Normal Grey Cockatiel) chicks and juveniles are phenotypically female, and virtually indistinguishable from the time of hatching until their first molting. They display horizontal yellow stripes or bars on the ventral surface of their tail feathers, yellow spots on the ventral surface of the primary flight feathers of their wings, a gray colored crest and face, and a dull orange patch on each of their cheeks.

Adult cockatiels are sexually dimorphic, though to a lesser degree than many other avian species. This is only evident after the first molting, typically occurring about six to nine months after hatching: the male loses the white or yellow barring and spots on the underside of his tail feathers and wings. The gray feathers on his cheeks and crest are replaced by bright yellow feathers, while the orange cheek patch becomes brighter and more distinct. The face and crest of the female will typically remain mostly gray, though also with an orange cheek patch. Additionally, the female commonly retains the horizontal barring on the underside of her tail feathers.

The color in cockatiels is derived from two pigments: Melanin (which provides the gray color in the feathers, eyes, beak, and feet), andlipochromes (which provide the yellow color on the face and tail and the orange color of the cheek patch). The gray color of the melanin overrides the yellow and orange of the lipochromes when both are present.

The melanin content decreases in the face of the males as they mature, allowing the yellow and orange lipochromes to be more visible, while an increase in melanin content in the tail causes the disappearance of the horizontal yellow tail bars.

In addition to these visible characteristics, the vocalization of adult males is typically louder and more complex than that of females.