Best lens for bird shots?

I've gotten hooked on shooting birds, having lived near a major flyway and belonging to Audubon. Anyway, I'm looking for suggestions on the focal length for getting shots of birds. Do you go with a teleconverter or go all out and buy a lens? 

I've been photographing for a long time and I've always loved nature shots. I used to go for landscape scenes but over the last few years I've enjoyed birds and other animals. I had a great time at a local Audubon place last summer. I sat in the middle of a trail watching a chipmunk and cranking off about a dozen shots. I didn't care whether anyone thought it was strange. That's what's always been great about photography - I just can forget everything else.