The sennies you just never know with them.
It's both a little annoying and fascinating. Oscar loves to be petted - especially when he's in his cage. He bows his head to show where to start and turns his head slowly around so you don't miss a spot. This can go on for minutes - mostly it's me who stops doing it, as he can't get enough.
In the same situation, he can sometimes suddenly flick his head and do a nibble that doesn't hurt. Or... a real bite that hurts. Most of the time I can read him, and see it coming, but sometimes, ahem.
He can even start to make the "throw up" movements and 3 seconds later give a good nip if he's startled by a noise, or just because he feels like it. He often follows it up with a sardonic laughter, which makes it very hard to be mad at him.
The only place he doesn't nip or bite is our faces. Eyes, lips, nose, he seems to understand that this would mean a real attack. But ears, he sometimes sees as a chewing target. And he clearly does it to get a reaction out of us; he loves to "test" his keepers.
Thing is, I can forgive most of it, as he's a wild animal that was brought up by humans, not a pet that has been bred for centuries to please humans. Most of all, he continues to baffle us with his intelligence. All coming from a tiny wee birdbrain.