I've been using the OpenCV library to build collision/avoidance sensors for UAV's. I think I can adapt the same code (with just a bit more sophistication) to discriminate between birds and squirrels if I use a webcam to image the feeder and its surroundings. Colleagues insist I should add a paintball gun to enforce the rule of law on the miscreant rodents. I think a simple chime of some sort would alert the cat or nearby humans to intervene. Damn Squirrels....

Invoking a Pavlovian response in cats would we a worthy reward for developing the BFSD in the first place!