Check out these filled nest tubes of Osmia californica, a late-spring-flying mason bee. The "plugs" are bright green because they are made of chewed leaves. Aren't they pretty?
The second photo is the setup for the mason bee introduction study I am doing with Mason Bees LLC. We have installed these in about 15 yards around Salt Lake City and are monitoring how the mason bees do in the boxes and what they are pollinating. Reeds on the outside of the box are those from which bees have already emerged; the inside of the box is filled with empty reeds. Looks like some females do prefer the used ones!