the bird

[chris1970]

why is giving the bird(sticking one's middle finger up )called giving the bird??

[Quizmonster]

Flipping is more common than giving...
'Bird', meaning the middle finger, is first recorded in Current Slang, published by the University of South Dakota in 1968. The only logical reason I can see for that is the fact that a hand with the middle finger raised might be thought to have a bird-shape...ie the head and neck outstretched like a duck in flight and the 'wings/body' formed by the other four digits.
The 'flip' element was added later to suggest the finger has imparted to it an appropriate up and down movement to simulate the activity it describes!
Click here and there are various blue headings to click on for further information.

[heathfield]

I've always linked 'giving someone the bird' as referring to an audience hissing or whistling to show their disapproval of an actor or speaker. Not knowing any better, I've always looked on the raised finger as a 'get it up you' gesture.

Amusingly, back in 1968, it became known round the world as the 'Hawaiian Good Luck Sign'! See here.

[Quizmonster]

That is, indeed, how it all started, H...originally the bird did, as you say, refer to an audience's goose-like hissing at bad acting.
The bird as finger concept, as in the question, however, came much later. When I mentioned in my earlier answer "an appropriate up and down movement", I was, of course, referring to simulated sexual - or, as you put it, "get it up you" - activity!