I don't know why, but this made me smile this morning.
You may have noticed a newish trend for retro-fitting external wall insulation to blocks of flats; 4" or so of mineral wool batts covered in a coloured render. Not only do they dramatically cut heating bills for those in old solid-walled apartments, they are proving very popular with green woodpeckers. Once they're through the render on a corner, it's short work to create a warm, snug, waterproof home to raise a family in at safe treetop level. And as birds learn very quickly from each-other, these are proving more popular than the harder work of hollowing out a burrow in a live tree.