Watch the whole bird, not one sign
Pinned eyes, a raised crest, fluffed feathers, leaning away, a tight body, tail flaring, crouching, or pacing can mean different things by species and situation. Read the whole picture before deciding what to do.
Relaxed birds look loose and able to choose
A comfortable bird usually moves easily, takes food gently, preens, explores, or steps closer on its own. The important part is not that the bird looks frozen and quiet; it is that the bird can move, pause, and choose.
Back up when the bird says no early
Leaning away, stepping back, freezing, tightening the body, lunging without contact, or refusing a favorite treat are useful answers. Respecting those early signs prevents bites and makes the next try easier.
Context changes the meaning
A sleepy cockatiel, excited conure, protective lovebird, singing canary, and nervous budgie may show stress differently. Body language is most useful when you compare it to that bird's normal routine.
Use food and distance as feedback
If the bird suddenly snatches food, drops food, refuses food, or takes it with a stiff body, the session may be too hard. Increase distance, lower your hand, use a perch, or end while trust is still intact.