Although I personally have only met one Cockatiel in my life who was probably impossible to tame down. I say probably because the woman who adopted him accepted him for what he was (a vampire) and got him a companion and wore gloves when she reached in to clean and feed them. It is very important that you understand your bird and clip his/her wings before being able to start the taming and training process, but given the proper time and attention, he may very well have been tamed down. As far as all of the others, the main ingredient in the taming process has been time.
First you have to ask yourself if you feel his quality of life would be improved by becoming tame. In most cases the answer is yes. Tame birds simply get more attention in most cases and, therefore, have better lives. However, some birds are perfectly happy to have another birdie companion and only need us humans for food and water! If you feel in his case his life would be improved by taming than I would begin by removing anything that may be adding to the aggression. Is he especially vicious in his cage? Most birds are but there may be things that cause extreme aggression. One of my tiels is the sweetest baby in the world until you give him a paper bag. Then he becomes Mr. Mean! Are there things in there that are contributing to his aggression? Nest boxes, mirrors and even some favorite toys can be major causes of aggression. They must be removed during the taming sessions (not all toys---only ones he may seem "overprotective over." This is why I say to make sure you feel his quality of life will be improved by taming. No one wants to take away a favorite toy even for a short time unless (in the long run) it will improve their bird's life.
Start by spending some time every morning and evening speaking softly and whistling to him through the cage bars. Does he come out of the cage on his own or at all? If not, there are a couple of ways to get him out. You could scoop him up in a towel but you must be careful not to harm him or scare him so that he harms himself running from you in the cage. (Never press down on the chest area of a bird while holding them in a towel. They don't have a diaphragm and can suffocate easily that way). If you feel comfortable toweling him, you should do that and take him in an unfamiliar room where he cannot see his cage and put him on the floor (never leave him unattended there). You should then begin speaking softly and whistling to him like you had been doing to him in the cage. You will then become the most familiar, comforting thing in the room to him and he may very well at that point "step up" for you. If he does, continue asking him to "step up" as though your hand was a ladder. Do this for a few minutes at a time, while speaking softly to him. Offer him favorite treats after this from your hand. Try to keep him from jumping to your shoulder. Your goal is to get him so familiar with the step up routine that you can eventually just go to him in his cage, show him the towel, then offer him your hand and say step up.
None of these steps will be accomplished overnight. The first few times you get him out of the cage he will probably be pretty freaked out. Give him only as much as he can handle and don't wear him out in any one session. Most birds get the picture about the towel fairly soon. They much prefer your hand to the towel.
Another way to go (an alternative to the toweling routine) would be to open the cage door and allow him to come out on his own. Then you could offer a perch to him and try the step up routine then. If he does, you would want to get him away from his cage and work with him further in a different room. All birds are easier to tame when they cannot see the security of their cage. Before you embark on any taming method, make sure you get his wings and nails clipped. It makes for a much "humbler" bird and makes taming much easier. I would also recommend that you have him health checked by an avian vet to make sure he doesn't have any underlying health problems making him "grouchy." Besides, the taming process is a stressful event for a bird and it's nice to know you are starting out with a healthy one. I hope this helps some. They are all individuals, and what works for one may not work for another, but this has been pretty successful for me.
Q. My Cockatiel isn't very tame, how can I tame and train him/her?
A. Are your Cockatiel's wings clipped? If not, that would be the very first and most important step. Even
if they are, he will still probably thrash within the cage when he sees the towel. Some birds don't take well to toweling and (since they are all individuals like us) sometimes you have to work with some birds differently. If he is really scared of the towel and you are afraid he will hurt himself running from it, it may be better to work with him gently by talking to him gently through the cage bars and trying to get him to come out onto a perch instead of using a towel. The only thing the towel is good for is getting the bird out of the cage. If you can get him to come out more peacefully and with less potential for injury, that is always best.
Once the bird is out of the cage, more training can begin to get him less afraid of human handling. Everything must be done gradually and in steps. In other words, the first night you try talking to him you can't expect him to step up onto the perch. You will need to gain his trust before you can go to the next training level. It will be up to you to decide when he is ready to offer the perch. Once he seems comfortable enough for you to stick the perch in the cage and offer it to him, he will probably not step up onto it right away. You may need to offer a treat simultaneously to get him to see the perch as a positive thing. Once he gets used to seeing the perch, and realizes it's not going to hurt him, he may step up onto it.
The next step then will be to take him into another room where he can't see his cage and work with him to go from the perch onto your hand. These sessions should be short and always rewarded at the end with a treat. Is your bird on a pelleted or seed diet? Are there any real favorite foods you are aware of? If not, start working on finding those for rewards. Also, you can try offering him a mirror as an incentive to step up (some birds — especially males — will go anywhere for a mirror). Every time you are victorious in getting him out of the cage, work extensively as possible on the step up routine. That is the key. If you can get the bird used to the step up command, that will help in getting him out of the cage and anywhere else. How does he come out now? Does he come out on his own? If so, that's fine, just use the time he is out to work on step up and always try to get him to step up from the cage before giving up and allowing him to come out on his own.
Q. My Cockatiel gets aggressive when he sees a cereal box, what can I do?
A. I, too, have a male tiel (Birdeeboy) who is sweet as pie until he comes in contact with a cereal box, soda can, or most any brightly colored container. He turns into Dracula almost immediately. The fortunate thing here is that his owner is smart enough to have identified what it is that triggers his aggressive behavior so that my hands don't turn into pin cushions when he goes into his frenzies. Your bird is also blessed with a smart cookie for an owner so that he isn't given away, labeled as "crazy," abused or shunned to his cage for what his owner may perceive as unprovoked attacks. In other words, your important work is done. You have identified the trigger. A lot of owners never learn to look for the triggers in aggressive behavior in pet birds and instead opt to take an easy way out, blame the bird for being "unpredictable" and then one of the above mentioned scenarios takes place.
Birds are, in many ways, still very "wild" as compared to our other animal companions (dogs and cats for example). Some of their behaviors must be accepted as part of their wild and wonderful personalities. Humans tend to laugh and show off their birds when they are found to be singing beautifully or prancing in a funny manner. Yet these same owners will shun their bird when they lunge at a hand coming at their cage for instance (protecting their "nest" or territory is completely natural for a bird). All of these behaviors are related to their natural personalities. They are simply being birds.
Now that you have identified the trigger (mirrors, certain toys) you have the upper hand. You can allow or restrict these items as you see fit. For example, when I am going to be out for long periods of time, I will give Birdeeboy his cereal box to look at. He doesn't even realize I am gone! He will sing and stare lovingly at it for hours. When I come back home, I (with gloves remove the object of his affection and, like magic, I get my sweet little Birdeeboy back. This can work to your advantage! Be careful, though, in allowing the bird access to his "trigger" for long periods of time as the aggressive behavior can become a more permanent part of his personality and become ingrained if this happens. For example, if you were to decide that he loved his mirror so much that you never removed it from the cage the bird may become "cagebound" (not want to leave the cage---his place of security) and could become antisocial and aggressive and need longer periods of socialization to get his pet quality back (however, I believe it is never too late to socialize a tiel).
Q. My Parakeet will step up but doesn't like her head-rubbed what can I do to further gain her trust?
A. In order to further gain her trust, you could try nightly sessions in a less familiar room (bathroom,
maybe, but make sure the toilet lid is down) away from her cage. Work on the "step up" routine to reinforce it. Talk to her softly and focus on her. Make this her "one on one" time with you. As time goes by begin to hold her closer to you as you speak to her. Be very careful that she doesn't nip your nose but try moving your nose closer to her gradually until you are able to gently rub her head with your nose. Once she gets used to that, you can wait until she is "tranced" enough to slowly move your finger up and have that take the place of your nose. You will be scratching her head in no time and she won't even realize it The first few times she comes out of her head scratching trance and realizes she is allowing you to scratch her she may very well jump away or nip, but in time she will realize that you aren't going to hurt her. I have found this to be a very good way to get birds to trust me enough to allow me to give them head rubs. Noses and faces in general are a lot less intimidating than fingers so it just seems to work out faster and easier. Just be very careful not to get bitten! It will take some time.