Found inside an old bird box I was cleaning out. Appears to be a nest of some kind of larvae. One dried up wormish larva thing barely visible in one photo; the rest was empty fibrous casings. The whole mass was spongy, dry, and incredibly strong--it was stuck to the wood of the bird box and very hard to remove. The larvae had also created small, oblong indentations in the wood of the bird box where the nest pressed up against it, though they don't appear to have actually burrowed in.