Petting my Caique, please don't laugh

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This is going to sound funny so don't laugh. I want to pet my Caique, well get her comfortable enough to let me pet her willingly. The thing is that it is a big deal for me, I had a parakeet for 15 years and he was my best friend and always let me scratch his head. He passed away a few years ago so I decided to get something more advanced. Since I was always good with birds I looked into adoption because I couldn't afford a baby, after a while of looking I eventually adopted a two year old caique. She was your typical give-up parrot, really mean and not trusting of humans. I have had her two years now, the first year and a half was rough with lots of biting and screaming. I really worked hard with her and am proud to say I have not gotten bitten in 9 months, I now pretty much completely trust her, she is even good with my wife. That said, I think getting her to let me pet her is very possible, its just another thing I have to teach her. When I try, she growls and very gently grabs my finger and moves it away, though she seems to let me get closer recently. Oddly, she does let me scratch her head through the cage bars. I know that birds are birds and they have their own twisted logic, so she may just not liked to be touched, if that is the case I will accept it. Anyway, does anybody have any advice on getting her comfortable enough to let me pet her. I don't want to push her so hard she goes back to biting, so any advice would be helpful. She is pretty well mannered otherwise and I am not afraid of her (Anymore). Its just a matter of her realizing that it feels nice to get your neck scratched.