Bird spikes would be the most effective. Energizing something to repel pests would require enough power/voltage to overcome resistance in the air to close a circuit. By then you may as well go up another notch and leave a platter, mashed potatoes, and gravy down there to have yourself some roast bird. I believe a nice Tesla coil may do the trick, I made one as a kid that would have roast the bird while it was still a few feet away.
