"O" is for "Osmia." Sexy, I know.
Osmia are mason bees - a very diverse group that includes 139 North American species. They all have a habit of plastering their nests with various substances: mud, plant material, sand, gravel, resin, or wood pith. Or flowers - check this!
Mason bees are easily managed in your backyard. They will colonize bee condos, which you can buy or make yourself.
There are several common Osmia species in Salt Lake City. Every year they come out in the same sequence, starting with O. lignaria ("blue orchard bee") in early April.
Followed by O. ribifloris, O. californica, O. montana, and O. texana, each 1-2 weeks apart.
All photo credits: www.bugguide.net (except the blurry one, mine...)