Basically one will collect pollen as far back as they can in the cell, and then lay an egg and cap up that part of with mud. They repeat this until the cell is filled. As usual with these insects, men are getting the short end of the tube. Males are specifically laid at the front of the tube. So in the event of a parasitic wasp, or woodpecker they're the first ones to get eaten. I'm not sure how active they are for the year but they're good for spring flowering plants. The developing brood matures over the summer and fall, but they don't emerge from their cells until next spring.
Video of one capping a cell.
http://smg.photobucket.com/albums/v735/mrilovetheants/Bees%202008/?action=view¤t=MasonBee.flv

