I've been hanging out around a particular group of feeders lately. I occasionally picked up a blurry flight shot by accident, and this weekend I decided to try for that on purpose:
This is natural light, no flash, Canon 7D + 400 f/4 DO on a tripod. f/8, ISO 800, 1/1600 s, -1 EV in AV mode. I've cropped and cloned out a couple of distracting elements.
From a nearly full 32GB card I got maybe 2-3 keepers. Focus was the biggest problem. You have to prefocus and hope the bird's in the right place when the shutter fires. I may try this again with a shorter lens to get more depth of field and a broader field of view, at the cost of losing some pixels on the bird.