The cockatiel is the only small parrot with a head crest.
The cockatiel’s head crest raises and lowers according to the bird’s mood. When the crest is raised to full height, it usually means that the bird is excited, fearful or curious. A lowered crest, with just the tip of it sticking up, indicates that the bird is content.
The cockatiel is one of the only small parrots that hisses and “spits” when frightened.