how do you focus if you wait for the bird to back up a little?
I use a tripod, set up right here next to my desk. The feeder is just outside my window. I can aim at the feeder while sitting at my desk. The sun hits the feeder just after noon, so I wait till then. The most important thing here is, I use a tripod to hold the camera.
So I point it at the spot the bird has been backing up to. It may take a few times, but when you get the timing right, just press the shutter half way when the bird backs into the open and in a few attempts, you will get it in focus. next time it backs into focus, push the button. Get the rythym going and you can press every time it backs into the focus area. One point, you are going to throw out 8 out of 10 shots, but with digital, it's free.
Another way I used last year, was to shoot with a zoom, zoom out so the angle is wide enough to get the feeder and half a foot of open space next to it. Shoot the bird as it backs away, the same as before, but you have a lot more space, which you will just crop bigger in post processing to get the bird looking close. You need a nice sharp lens for this, but it does work.