Flipping the bird

Where does the phrase "flipping someone the bird" come from?

I couldn't find anything on the use of "bird" for finger in this phrase.

FYI

Then he shot me a bird -- flipped me the old one-fingered international salute. (_The New York Times Magazine_, Nov 22, 1970)

They flip him the bird, and he lets them pass. (Lucas et al., _American Graffiti_, 1973)

She'll never forget seeing her son flipping the bird in front of 40,000 people. (Lyle et al., _Bronx Zoo_, 1978)