[SDavid]

"I also have learned they are disproving this theory also "

The "theory" was probably just bird owners guessing. I think the simple truth is that birds feel safe and secure when they are up high because they are prey animals and anxious and fearfull when on the ground. For one example, their crop enables them to gulp down food quickly and take to the sky asap. To stay and chew food would keep them on the ground and in danger for too long. For another example just look at the primary (probably only) defense mechanism birds have evolved-when there's danger they take off. Up and away. They don't stay and fight (they're too fragile) but escape. Even when defending their young they bluff and try to frighten a predator away. So, I'm sure sitting up above everything gives them a feeling of confidence and safety.

When my TAG is on top of her cage she usually makes a fuss if you try to remove her. But, left alone, she seems to be having a ball vocalizing and climbing around.

"If I have an aggressive bird I can reduce the aggression by lowering the cage and perches or putting the bird on the floor. "

I'm not a fan of that idea. I don't know if putting a bird in "fear" is a good way to reduce aggression. Even if it works I don't think the end justifies the means. And, a bird in fear is too risky for other bad behaviors depending on how sensitive the species is. I've never had to deal with aggression but I think there are better ways. For one, Melinda Johnson's "clicker training," or any method that makes use of ABA.