to other cockatoos is further supported by the existence of at least one documented case of a successful hybrid between a cockatiel and a galah, another cockatoo species. Description The cockatiel's distinctive crest expresses the animal's emotional state. The crest is dramatically vertical when the cockatiel is startled or excited, gently oblique in its neutral or relaxed state, and flattened close to the head when the animal is angry or defensive. The crest is also held flat but protrudes outward in the back when the cockatiel is trying to appear alluring or flirtatious. When the cockatiel is tired, the crest is seen positioned halfway upwards, with the tip of the crest usually curling upward. In contrast to most cockatoos, the cockatiel has long tail feathers roughly making up half of its total length. At , the cockatiel is the smallest of the cockatoos which are generally larger at between . The "normal grey" or "wild-type" cockatiel's plumage is primarily grey with prominent white flashes on the outer edges of each wing. The face of the male is yellow or white, while the face of the female is primarily grey or light grey, and both sexes feature a round orange area on both ears, often referred to as "cheddar cheeks". This orange colouration is generally vibrant in adult males, and often quite muted in females. Visual sexing is often possible with this variant of the bird. Cockatiels are relatively vocal birds, the calls of the male being more varied than that of the female. Cockatiels can be taught to sing specific melodies, to the extent that some cockatiels have been demonstrated synchronizing their melodies with the songs of humans, and speak many words and phrases. They have also learned to imitate certain human or environmental sounds without being taught how to do so. Sexual dimorphism Most wild cockatiel chicks and juveniles look female, and are virtually indistinguishable from the time of hatching until their first moulting. 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 grey coloured crest and face, and a dull orange patch on each of their cheeks. However some modernday mutations are sexlinked and the male and female chicks are easily distinquishable as soon as their feathers come in. Adult cockatiels with common coloring (grey body with