Something is making holes in the sides and tops of the lead plumbing vent pipes on our roof. They need replacing again after about 1 1/2 years because each time something apparently gnaws holes in the pipes - big holes - about 2 to 3 inches high and a third of the way around the pipe. The holes are close to the bottom of the pipes. Is there anything we can do to prevent this? What could be doing this - squirrels, birds?