Apprentices seem to be getting this better now than they did a few years ago. There's a rule of thumb that claims the passage bird's response weight is 10% lower than its trapped weight. I've seen people take this rule as law without taking into account the condition of the bird. This generally happens with those who can't be bothered to carefully observe the bird's reactions and behaviors at different weights. I've seen it in generals and masters as well.

A friend of mine trapped a redtail at 32 oz. Its weight implied the bird was a tiercel; being an apprentice he had no experience judging its physical size. At that stage in the game ANY bird will look huge and magnificent. If he'd followed the 10% rule blindly he would have killed her because she was actually on her last legs -- no breast, major infection. Her head size declared her a female, and a pretty good-sized one at that; her field weight seems to be 38.

I've recently had the experience of knowing some falconers who will look at a bird and declare a flying weight. I've heard of a falconer advising another to take an eyas bird down more than 10% in order to get it to learn to take ducks instead of jacks. This is insane. NOBODY can specify a bird's weight just by looking. The muscles must be felt, the behavior under different weights observed, and even then it can only be approximated.

'Weight control' as used by Harry McElroy (and other master falconers) is the method of maintaining a bird at a specific weight throughout the hunting season and not deviating from it by more than a few percent. This term is frequently abused by certain falconers to justify their starving a bird into submission.

If you fly your bird too high, the worst thing that will happen is you lose her. If you fly her too low, you can kill her.