Females are in charge of building the nest. The male follows the female closely as she forages and builds the nest. The male is close by both signaling her when it is safe to fly out of the nest box and to prevent other males from mating with her. A nest normally takes about six days to complete. All of this is dependent upon weather and individual bluebird habits.
Pairs normally spend less time building nests for their second or third broods.
During egg laying, the female leaves the eggs unattended and begins incubating after the last or second to the last egg is laid. She is also in charge of incubating the eggs by herself. One reason for this is that she develops a brood patch consisting of an area on her breast where feathers are lost and blood vessels increase. This allows her to keep the eggs at the right temperature for development. You will want to monitor your nest box to make sure house sparrows and starlings are not competing with the bluebirds for the nest.