Nest Box Casualty

It's getting near the time to clean out the nest boxes in advance of winter: in case birds seek to use them as refuges from the wind. I've been giving cursory inspections of the "fleet" (there are nine out there) and discovered some late season damage. This one looks like the casualty of my buddies the squirrels. The chew marks are pretty distinctive. The retaining nail wasn't in all the way--it seems--but even had it been so the squirrels would surely have gotten in. This box will be retired unless I can coax out an entrance guard with my limited workshop resources. Too bad: I don't know if this was a nest that almost was. 'Twas our first known casualty of the season!