Why does my bird scream?
First, you will need to establish whether your bird is really 'screaming' or whether your parrot is simply vocalizing. A very excited happy parrot can vocalize in a way that may seem like screaming to humans, but in bird terms, is merely sounding like a parrot should.
A bird's environment also has a lot to do with how vocal they are. Sun conures have a reputation of being very loud birds. Although a lot of them are, we have one little sun who boards with us and he has the quietest, softest voice I have ever heard from a sun. He lives in a home with two older adults who are rather quiet themselves. So keep in mind if your household is a rather busy noisey one, then your parrot will mirror this. Kids playing and running around, lots of conversations going on, loud tv or music, all tend to make a parrot want to join in on all the excitement. So if you want to quiet your bird, try quieting it's environment first.
Many parrots are rather noisy first thing in the morning as they greet the day, and often in late afternoon or early evening as they also end their day. These types of noise sessions usually only last 15-30 minutes, but as the owner of amazons, I can personally testify that my amazons seem sometimes to end their day with an hour long yelling session.
Although this type of 'screaming' may be very annoying to the humans, it is natural for the birds, and you will probably never completely remove these sessions. Sometimes they can be shortened by encouraging the bird to sing or whistle instead of using the loud parrot yell, or even a treat might help shorten the session. Just be careful not to let the bird know yell=treat every time or you will only train the bird to train you better.
Speaking of which, if your bird screams every time it sees you eating something it also wants, well then you are probably well trained. To retrain, it is good to only give the treat before the scream can begin. So if you intend to get yourself a little snack within view of your bird, you might do better to give your bird his or her own treat, before you go and get yours. Be sure to also verbally reward your bird with lots of praise for being so good and not screaming for the treat.
Some birds seem to scream and yell every time their owners gets out of view. As flock birds, parrots want to make sure you are still somewhere near and haven't been abducted leaving them all alone. Sometimes just letting them know you are still near in a calm voice will help. You may have to gently shout back to them every few minutes some days, and others maybe only once or twice. One of my cockatoos is this type of yeller. At times she sounds very pitiful and whinney in her yells, and then there are the days when she sounds downright mad as heck because I'm missing in action and she just KNOWS I'm somewhere within screaming distance. The sad yells I answer back and try to reassure her I'm still around. The angry screams I try really hard to ignore and walk away. Yelling back angrily at this type of screaming only gives the bird your full attention and usually angry humans are rather amusing to our parrots so it can be quite fun for the parrot. Most parrots will eventually quiet down and then you can go and praise him or her for being such a good bird. Sometimes it works, and sometimes those cockatoo screams can be quite unnerving requiring super-human strength, or at least a good set of earplugs, to outlast.
There is also the warning 'scream' that many conures and quakers are very good at. I often tell people who purchase a conure, that if they put the bird in front of a nice window, their sweet little bird is pretty much guaranteed to let them know (and very loudly) every time a squirrel crosses the yard, a flock of birds pass by, or a neighborhood stray dog or cat intrudes into the yard. Conures especially are very accurate watch birds. So, if your bird screams a lot in front of the window, then duh, maybe you should consider relocating the bird somewhere else for at least part of the day.
Birds also can scream or yell because something frightens them. If your bird appears to be yelling for no apparent reason, then you should closely inspect the surroundings to see if something is out of place. The other day I accidentally dropped the lid off a container of bird food. Since no birds yelled out the "invasion scream", I didn't immediately pick it up. About a 5 full minutes later however, there was such a ruckus of conure screams to deafen the whole house, and that's putting it mildly. So even if you don't find the scary item right away, keep looking and trying to figure out the warning message. Even moving a piece of furniture or a plant across the room can do it. Sometimes we humans are a little slow to the show according to our birds.
If you have a bird that screams when someone they don't know enters the room, you probably have a very nervous bird who is screaming out a warning. You will need to work carefully to help your bird become less afraid. Keeping strangers away from the cage can help, making sure the newcomer puts their hands behind their back when approaching the bird may also help. If the bird has an unknown background, then there may have been mental or physical abuse by someone in it's past, and it may take a very long time before the bird begins to become more at ease with other people. Depending on the abuse, there is also the possibly that your bird may never feel at ease with most people, and you would be best to not try and force it. Respecting the bird's fear and keeping people it is not comfortable with away from the bird is probably the best thing to do. Some birds simply never overcome the trauma of the past 100%.
If your bird screams at only one family member, then there is something about that particular person that frightens the bird. If the person is a child, then the quick movements of a young person may be the cause. If that is the case then explaining to the child why the bird is upset, and the best way for the child to act around the bird, can often turn the dislike into curriousity by the bird of the little human in it's world. Person aversion can also sometimes turn out to be something as simple as a hat or cap they wear, eye glasses, certain colors they like to wear, or something that can easily be changed once you figure out the cause.
Also, always rule out a health issue or pain from an injury if your bird begin screaming when he has not done so before. Blood feathers, overgrown toenails, and such can cause pain that the person may not notice at first glance.
Some people believe in "time-outs" for birds who scream and will cover the bird's cage until the bird calms down. I have decided not to really go into this type of therapy as in some cases it might work for a bird, and in other cases it might actually cause more harm than good. I have seen cases where people think it is ok to cover the bird and then they walk away and just forget about the bird. In these cases the bird probably thought it was bedtime and after a while went to sleep. There was no real link in the bird's mind between the action and the reaction by the human. If you do decide to cover your bird for being out of control, do so for only a few minutes and then uncover and praise the bird.
One last screaming scenario I'd like to mention is the bird who screams after it is put to bed. If the bird is uncovered then it would be logical to try covering the bird to see if that improves the sleep time. Some birds really want a dark environment to rest best. There is covering the cage, and then there is 'really covering the cage'. I once had a bird owner with the screaming at night problem who swore they completely covered the bird's cage and it still screamed and screamed. After several months of trying to work with this owner to figure out why the bird was even now beginning to scream during the day, they finally reached their limit and begged me to take the bird. The situation was causing a problem between the couple as the bird was also becoming aggressive. The bird arrives with it's cage, and it's cage cover. The cover turned out to be a towel that the owners threw over the cage at night. The towel cover only covered the top and maybe 2/3 of the sides of the cage. In other words the bird was not really fully covered at night. The first night with me I changed the cover to a large dark sheet that completely covered the top and all 4 sides of the cage. I never heard a peep from the bird after the cover went on. Because the bird was now properly covered and sleeping much better at night, she was now a well rested happy little bird and has never bitten anyone since.
Trying to quiet a noisy bird can be quite a challenge in some cases. Begin by deciding whether you truly have a screaming bird or just a very happy vocalizing bird. If you do truly have a screamer, then move on to working the problem out as best you can. It is never ever ok to strike a bird for any reason and you may do great physical harm, or even unrepairable mental damage. Trying to out-scream your bird, well that's just a silly new game for your parrot to enjoy. Just remember that some birds are not the quiet type and you might just have to learn to live with a little noise now and then.