GCCs mature sexually between 1-2 years (some say they don't usually breed until 3, but that seems very late to me). If you have ruled out medical issues and have taken your bird to the vet to verify that the change is not medical, then I would lean towards some other possibilities like hormones, sleep (which relates to hormones, behavior and immune health) and possibly diet. These things are all closely interwoven/overlapping, although one may be the primary (it's hard to say, which is why all must be addressed)
**It is always good to get a certified avian vet's opinion when there is a sudden behavior change**
Also- how often is your bird out of the cage, and does your bird know how to play with the toys it has? You often have to teach them if they have toys and ignore them, or try new toys with different thicknesses, materials etc (only after making sure they are comfortable with them, before putting them in the cage)
You (any anyone else) should only ever pet on the head and neck and should not cuddle them-if you do, it's very sexual and stimulates them, but also makes them view you as a mate and as a mate who is not following through, you can sexually frustrate your parrot and increase anxious behaviors, like screaming, plucking, biting etc...because you actually can alter their hormone production this way too. Yes- they like to cuddle, but you have to be the one to draw the line and they will be happier for it in the long run (babies/immature birds will not respond immediately/in a sexual way to these types of touches etc, but the problem is, it sets an expectation and still frames the relationship, so suddenly, at puberty, what seemed innocent no longer is, but then they really want it, because it's hard-wired into them to pair off and mate when conditions are favorable )..even if this behavior isn't just hormones, what I am telling you about head and neck petting only, minimizing access to shadowy/ dark spaces etc applies to all parrots.
They MUST have at least 10 hours of quiet sleep each night on a routine, as this helps regulate mood, hormones and immune health. It's super important that they have a sleep schedule.
If you have been allowing your bird access to snuggle huts, don't. Snuggle huts are extremely dangerous for other reasons as well, but definitely contribute to hormones in the vast majority of users...same with tents, coconuts, under tables, in bedding/blankets, piles of paper, shadowy spaces etc. They should have no access to shadowy spaces as these spaces are preferred by them due to nesting tendencies and the presence of such spaces amplifies their hormonal response/triggers them and can also make them more territorial/cage aggressive.
You need to make sure you teach independence by doing things gradually, but one thing is to NEVER come back into the room while they are screaming. If a bird screams for attention, don't give it a single reaction-- not a verbal reply, not eye contact, not proximity. To prevent screaming, there are some other tricks and I will post a link to a thread- read over this, but especially the counting technique that I discuss in my 2nd reply:
http://www.parrotforums.com/cockatiels/88181-cockatiel-screaming-if-he-s-not-out-his-cage.html
When you leave (BEFORE THE SCREAMING STARTS), do you tell him what you are doing? Key words can be huge in helping them gauge time and anticipate things. Whenever I go to the bathroom, I say going to the bathroom" --if while I'm in there, she makes a cute sound, like talking or something, I will talk back, but never respond to attention screaming once it starts. As another form of practice, you can start stepping away for just under the amount of time you think it will take before the screaming starts, saying "be right back" and then BEFORE the screaming starts, return and say, "thanks for being quiet" (praise, treat, whatever your bird is most motivated by).
Diet should be pellets, some seeds (no sunflowers), chop etc (more veg than fruit).
When it comes to the attacks- please tell me what you are doing immediately before and what you (and anyone else, including other animals in the room) do when the attacks occur, and right after as well (including any talking, moving, eye contact, walking away, people/animals coming into the room etc)
