I think this will be my last update on this thread. Around a week ago Shrek stepped up on my hand for the very first time. Since then he has become bolder and more outgoing. I've been taking him outside while I do my gardening and chicken chores. He likes to run between the flowers and jump out at my hands while I'm pulling weeds. He reminds me of a precocious, aggravating little boy. ...But a very lovable one. Last night i was sitting with him as per our nightly ritual. He came close to my face and pressed his beak against my cheek. I hadn't tried to pet him before this but it just seemed like the moment for it. I stroked his chin and he bent his head down and let me rub the back of his head and neck. I'm not much of a crier but I'll admit I let a tear slip.
I really am surprised by the progress Shrek has made in such a short time. It's hard to describe just how traumatized and angry he was at first. The vet tells us he's gained a healthy weight and the vitamin deficiency must be resolved since his feathers are growing in beautifully. He's even starting to get his ring.
Thank you all for the advice and encouragement. This forum and the people here have been a wealth of wisdom and help. I know Shrek would not have come so far without the advice we've received from our bird friends.