Upon pulling away several segments of owl pellet, I found these four beetles underneath. I see now why Phillip Harpootlian says these beetles are "often encrusted with dirt." It makes sense, though. In the cool early Spring they were staying under the owl pellet, but to get their mouthparts on it to feed they had to turn their back to the ground. The ground was pretty muddy, so their backs get covered with the mud.

It's possible on the coldest days they actually burrow into the soil for shelter——that would be something interesting to check on.

I remembered to take a measurement of these (I brought home a twig I had pinched off at the proper size) and they measured 8 mm long, pretty small. Unfortunately that rules out only a few species of these Troginae beetles. Anyone who can help with an ID please chime in; the encrustation hides some of the features, alas.