I'll say this then I'll go. Serious bird flipping is serious business. Someone who knows how to do it properly will scare the poop out of you. It's usually accompanied by cursing and threats of immediate death. That's the way to scare someone.
That happened to me one time out fishing in the Gulf of Mexico. Stupid thing, but I went out with a newbie (he wanted to go snapper fishing in his new leaky boat) and he drove right up on some guys from Biloxi about 30 miles out. We were gonna drop anchor and fish. I told him we were approaching too fast (bad fishing ettiquette). There were two guys and a woman there and they weren't fishing. Their boat was about 24' cuddy cabin.
One man said, we drop anchor there they would kill us. Looked like he had a revolver in his hand. The other one flipped the bird in a way I'd never seen before, and was ectremely intimidating.
We got the hellout of there fast.
Les