I drilled some holes of varying sizes in a funky piece of wood that my dad had laying around from chopping wood, with the hopes that mason bees would nest in them, and it's been a rousing success! The mud capped nests are from long-horned masons (Osmia cornifrons) who were active earlier in the season. Right now the Georgia masons (Osmia georgica) are out; they make a paste from chewed up leaves to partition and cap their holes. The other morning when it was a little too chilly for them to be flying, I saw 6 bees peeking out at me. There are also a bunch of little solitary wasps who are living in the holes. They haven't capped any yet, and they've been flying longer than the Georgia masons have, so I'm not sure if they just don't fill the tubes all the way or if they aren't actually nesting in the holes. I haven't been able to ID the wasps yet.