Western Bluebirds are attracted to habitat such as open forests, parklands with scattered oaks or pine trees, orchards where there is no pesticide spraying, or large mowed lawns with mature trees. They also do well in urban parks, golf courses, and even cemeteries. These open areas with low vegetation allow them to easily see and hunt insects. Location is important. Place boxes in open spaces with scattered trees. If you put up a nestbox in an area of dry or dense shrubbery and trees, you will get wrens, flycatchers and titmice, not bluebirds.
Ideally, the nestbox should have some trees or shrubs nearby to provide perches from which to hunt, and for the young to fly to when they are ready to fledge. The entrance of the nestbox should face away from prevailing winds and rain. In San Diego County, California the opening is best oriented north-east or south-east. And placing a box in the shade or partial shade of a tree helps reduce the temperature inside the box during the hot summer months.
When breeding, Western Bluebirds have territories of about 2-3 acres and will protect and defend them from other bluebirds. Western Bluebirds do not usually allow other bluebirds to nest within about 100 yards, so space nestboxes accordingly. To reduce territorial fighting, it is better to space nestboxes farther apart than too close together; and place them so that the occupants can't see one another.
There are many different designs for bluebird nestboxes. Choose a nestbox that is made of wood for good insulation from heat and cold, and has an entrance hole of 1-9/16 inches in diameter to prevent non-native European Starlings from using the box. The diameter of the interior floor should be at least 5 by 5 inches for Western Bluebirds. And there should be about 7 inches from the bottom of the entrance hole to the floor to keep eggs and nestlings out of reach of jays or crows that might poke their heads into the nestbox and try to snatch them. If the wood of the box is very smooth, it is a good idea to cut grooves into the wood below the inside entrance hole to help nestlings get a foothold as they attempt to fledge. The wood (cedar or pine) should be at least 3/4 inch thick to provide adequate insulation from the sun. If you live in areas of the county where summer temperatures are regularly over 90 degrees, then you should add 1/4 to 1/2 inch ventilation holes in the top of the sides of the box; or design a box with 1/2 inch space gaps between the top of the sides and the roof; and consider placing the box where it will get shade from the afternoon sun. The roof should overhang the entrance hole at least 1 to 2 inches to keep out rain and shade the entrance. In order to monitor the box, it will also need to open either from the front, side, or top.
In San Diego County, we don't have a lot of problems with climbing predators like raccoons or opossum. That's because the riparian habitat that supports these animals is disappearing, and also because we still have large predators (mountain lions, coyotes, bobcats and foxes) which help keep their populations in check. Nor do we have climbing black rat snakes, which are so prevalent in the south-east. So hanging a nestbox in a tree here is not the "invitation to disaster" that it might be in other parts of the country. Nestboxes that have been hung in trees for years here might only face problems with ants or earwigs, which can be controlled with Tanglefoot. Trees also provide needed shade during our hot summer months. However, mounting the box on a pole under the shade of a tree at a height of 4 to 6 feet also works very well; and is an easier way to control ants with a Tanglefoot guard.
Slot-boxes allow a bird cornered in the box (by a House Sparrow) to escape and survive. Slot-boxes also provide good air circulation; and do not require an entrance hole drill for construction. However, the wider opening does make it easier for scrub jays, crows, and Cooper's hawks to poke their heads in and snatch nestlings.
Nestboxes can be put up any time of the year. In the fall the bluebirds will locate potential nest sites for the coming year, and during the winter they are often used as roost boxes for protection from winter weather. In the spring and summer, bluebirds will use the boxes for nesting from late March to the beginning of August.