The beak is a birds way of communicating... either by noise or touch.
Casper knows exactly how hard to bite in order to draw blood. But he stopped doing that when he got us trained properly
Fortunately he doesn't bite that hard any more but we will occasionally get a nip if he thinks we deserve it (but for reasons that are not explained to us).
We also get nips when we are not paying attention and he wants neck scritches, or he attacks our hands when we are trying to type, or if he is on the floor wanting to be picked up he nibbles at our toes.
And then there is boisterous play, he loves to bite the webs of fingers.
None of it really hard and if he does hurt us he gets told off and put back in his cage. Which should make him realise we are not pleased but sometimes when he mimics the telling off it is hard not to laugh.