That's wild, dude. Is it lethal, or just a significant reminder to its guest that popping in again probably isn't a good idea?
I've never seen one of those.
Wrens and Red Headed Woodpeckers are the biggest problem for our Bluebird nesting. They're tough little rascals though. And the boxes are close enough for me to assist in the deterrent effort. Even so they abandoned two nests this spring -- mostly due to the hassle from the Red Heads. First year I've seen that behavior from them. The wrens usually just get an ass-kicking and eventually abandon their efforts to occupy a box.
Good on you guys lookin' out for em in your areas and doing the cams. I'll get one going here sooner or later.