I think it associates the movement with food, I just started doing this little wave thing with my fingers whenever I went in to feed it and it responds now. Dumb luck I guess, but it wasn’t something I intended to do, it just happened.
I used to have a turtle who knew the sound of my feet when I would walk up the stairs at a certain time every day, after work. He knew he’d get fed, and would jump off his rock and swim around, with his face against the glass. At other times of day, he’d just stay on his rock despite my weight coming up the stairs.
I have a little sparrow girl now, who chatters every time I walk past the door to her room. She doesn’t chatter when my husband walks by that door.
They “know” you in more ways than just through sight - they know your footprints and vibrations; like a horse senses when its rider turns his head to look left or right...