A trainer's small movements can be big to the bird. In addition, the bird's small movements, to a sensitive trainer, are also big. Think about the reaction one can get just by looking at an aggressive hawk. The slightest movement of a trainer's hand can be interpreted as a cue to a bird. For a parrot, a hand moving to go into a pocket for a treat could bridge a behavior and end it right there.
Birds are incredibly perceptive.
It is important for trainers to challenge themselves to be as perceptive about how their behavior is affecting the bird's behavior. Trainers can look for small changes in bird body language to cue themselves something is going on that is effecting the bird's behavior. Certainly many trainers have had a bird stop performing a behavior and look up. The trainer looks to see what the bird sees and there is a little tiny speck of an eagle soaring overhead. But trainers also need to focus on even smaller behavior changes.