So, I've cleaned out all three boxes I have. Two of them I'm not sure were actually used, though there were twigs in nest form (and not just masses of twigs like housew wrens do with boxes they're not using), but not lots of feathers or whatever.
In the third box, which was used by tree swallows (seen here in May; if you look closely, you can see one inside):
In July after they'd fledged, I looked at the nest box, but didn't disrupt the nest in case another pair of birds wanted to use it, and this is what I saw:
And when I cleaned out the nest, I took pictures of the feathers to show how big they were (and also, you can tell that I didn't take them all, and that they look mostly in worse shape. Next time, I'll check after the fledging and take better pictures!)
I've used a US quarter coin for size comparison. (And I returned the feathers to the yard on the chance that they're raptor feathers.)
It's impressive to me that such tiny birds find and use such big feathers in their nests!