hes doing it cause of the noise it makes. its mating season right now and it is the males that "pound pecker". the louder the noise, the stronger the male and the chicks dig it. he's goin for the metal cause its louder than wood. sort of like stuffing a rolled up sock down your pants. shooting him is pretty drastic when all you have to do is wait a week or two. just leave the cat on the roof for a while, till mating season is over.
or muffle the metal, wrap some foam around it, no more prob. - if it dont make noise he wont hit it.