I have to share my admiration for those who adopt an animal with already known behavior issues. We find this often with parrots...after all biting, screaming and destructive chewing are among the top reasons a parrot is surrendered to an animal shelter.
It takes dedication and commitment to live with any animal, and it's one thing to have your precious baby bird learn how to bite you for effect over time, but to start off with one that has already tried to take your eye out, or remove a finger, takes that commitment to a whole new level. You have to set aside any expectations that didn't involve getting lunged at every morning while offering food. I'm not going to question sanity, as it would bring into question my own, but I am going to be impressed with the compassion and understanding it takes to start out.
Many biting and screaming problems in birds have been taught, though perhaps inadvertently, by a former environment. We all know how easy it can be to push a step too far, and we are trainers with experience. We know how much easier it is for new students to ask for too much, too soon. We have seen extinction induced frustration and perhaps aggression.
Often, with positive reinforcement we can mitigate the effects of mistakes by establishing our criteria a step back, creating a history of reinforcement and moving up carefully from there. Step up does not need to be the first behavior we work on. Targeting can be, and with great effect. However, when you take a bird that has decades of experience with force, there is a challenge to integrate them into a friendly part of your family. Perhaps we end up not ever meeting that goal, and perhaps with perseverance and luck we do. Bit of a gamble in any case and we need to acknowledge and document progress when it occurs.
Nothing brings these lessons closer to home than when I foster a bird in my home that has been forced to perform behaviors using aversive methods. Great timing and attention to detail can make major changes in a short period of time. Every moment is a learning opportunity for both of us. If a bird is trying to squeeze itself through the cage bars, beak first, Matrix style, then much of my training is spent doing what I call 'drive-by training'. Humans in general can be an aversive to some birds and obviously standing in front of the cage trying to train a target while the bird lunges at you is not advised.
Instead, I systematically discover favorite foods, and once identified, drop them into a food bowl in the cage and promptly walk away. Once the bird begins sitting calmly when I approach, I work up to offering a treat by hand (carefully!) through the cage bars, then walking away. Then I proceed to increasing the time I spend in front of the cage, usually engaging in targeting... first a simple stretch, then a step, then movement all over the cage.
This is huge and measurable progress! It occurs without any injuries to me and at the bird's pace. It begins that relationship of trust that is built, step-by-step, by creating positive experiences through positive reinforcement. It's an excellent place to start.