This very small bee was exploring around the various nest holes in this bee box. You'll notice the greenish 'pack' on its thorax? No idea what that is, but it looks like pollen. I asked a bee expert what it might be and she also thought it might be pollen. Since taking this photo, I've seen a honey bee (Apis) with a similar pollen pack on its thorax. This tiny bee is in the Tribe Osmiini, and in the genus Heriades (with a thanks to Bug Guide for identifying the genus).