We had them attack a few of my hive a couple of years ago, we have put protection on for years before with no sign of trouble but left it off one year, all was ok until it snowed then they struck, splintered the blocks out then started on the front wall of the hives luckily we just popped in on the way to somewhere else and caught the yaffle (green woodpecker)in the act before to much damage was done,(on two different sites), they say this is a learned behaviour and we have found they only seem to attempt this when times are hard, we know insure that we protect with netting or chicken wire on all hive where we know the yaffles frequent, the image of the damage in the op did look quite similar to what i had experienced so its a good chance it was them, if in doubt protect better safe than sorry. chris