How to Understand Cockatiel Gestures
To understand if your cockatiel is happy, watch for it to wag its tail from side to side. Additionally, you will know your cockatiel is happy if it walks towards you with its head up. alternatively, your cockatiel might be angry if it puts its head down, ruffles its feathers, or flips upside down with its wings stretched out. To find out if it's seeking attention, look for the cockatiel to bang its beak against the cage or counter top. Other attention-seeking behaviors include hopping or moving the head from side to side.
Notice tail wagging. Birds do wag their tails, just like other pets. The bird will wag it back and forth. Generally, this gesture means the bird is happy. See if he walks toward you. If you're nearby and he comes to you, that's a sign he's happy you're there. However, he's only happy if his head is up as he walks to you, rather than pointing face-down. Listen for noise. While generally not a gesture, cockatiels like to talk when they are happy. They will sing to themselves or whistle. They'll also make little chirping noises. Pay attention to flashing or dilating pupils. If the cockatiel's eyes suddenly dilate, that could be a sign he's getting mad. Stop what you're doing if you see this warning sign. Look at his head and feathers. When he's really angry, he may put his head down. He may also ruffle up his feathers, and fan his tail feathers. If he starts walking toward you in this position, he really wants you out of the way. See if he flips upside-down. This stance, paired with spread wings, is usually a sign he's trying to defend his territory. It's probably time to back away if he's making this stance while you're near the cage. Watch for snapping. Cockatiels will lunge at you if they are about to bite. They may also snap in your direction with their beak. You should leave him alone for a bit if he's trying to snap at you. Listen for hissing. While hissing is not a gesture, it does go hand-in-hand with other aggressive behavior, such as lunging. If you hear your cockatiel hiss, he may be about to bite. Notice wing flapping. Wing flapping, when the bird makes an expansive gesture with his wings and moves them up and down, is usually a sign he's mad or annoyed. Try to leave him alone for a bit, if you've been bothering him. Look for beak banging. Some cockatiels, usually males, will bang their beaks against things, such as counter tops and cages. The idea is to be noticed, usually by whatever or whoever has become their love interest. Cockatiels can fall in love with objects, their own reflection, other birds, and even you. He may also whistle or lean towards the person or item. Watch for hopping. Hopping is similar to beak banging; he's looking for attention. However, hopping is taking it to the next level from beak banging. He's really begging for attention. Listen for loud squawking. Sometimes, in coordination with other behaviors, cockatiels will make loud squawking or screaming noises. Basically, the cockatiel is looking for attention. Notice head snaking. Head snaking is when the bird moves his head from side to side. It's a fluid motion, not jerking. Usually, the bird is just asking to be noticed. Look for the cresting of the head feathers. When a cockatiel is trying to attract a mate, he may let the feathers on the top of his head curl up into a crest. Basically, it creates a little curlicue on the top of his head. However, he could also be defending his territory with this gesture. Watch the tail and wing feathers. Another sign of seduction is the fanning of the tail feathers, paired with the cresting of the head feathers and the spreading out the wings. He may also strut around and whistle. Again, this gesture could also be a sign he's defending his territory. Watch if he hisses. This is usually a sign for you to back off. Watch if your cockatiel bobs his tail. Sometimes, when a cockatiel is sick, he will bob his tail. If you notice this sign, it may be time to take him to the vet. Look for sitting. Another indicator your bird could be sick is that he sits. He'll either hunch down on the perch or sit on the bottom of the cage. Notice other signs of sickness. While these signs aren't necessarily "gestures," they can also indicate sickness. For instance, your bird may sneeze, get extra sleepy, or lose his voice. He may also may eat more or less, or he may suddenly drink more water. Finally, his droppings may change in appearance (color) or number. You might also notice behavioral changes in your bird. Feather plucking may be sign of boredom, but is also a symptom of Beak and Feather disease 
Notice tail wagging. See if he walks toward you. Listen for noise. Pay attention to flashing or dilating pupils. Look at his head and feathers. See if he flips upside-down. Watch for snapping. Listen for hissing. Notice wing flapping. Look for beak banging. Watch for hopping. Listen for loud squawking. Notice head snaking. Look for the cresting of the head feathers. Watch the tail and wing feathers. Watch if your cockatiel bobs his tail. Look for sitting. Notice other signs of sickness. You might also notice behavioral changes in your bird. 