I put up one of these and have hosted Western Bluebirds nesting every year since. In the wet years they do 3 litters back to back, dry years with less insects to feed babies, one batch.
I learned a lot from https://socalbluebirds.org and I have some of their houses which are really good when available.
Nature's Way Cedar Bluebird House with Viewing WindowMonitor bluebird nesting activity in your yard by placing this Nature's Way Cedar Bluebird House with Viewing Window in it. The classic shape and sturdy, cedar body form this robust shelter, perfectly designed with bluebirds' needs in mind. A 1" thick predator guard surrounds the 1.5" diameter entry hole, creating a physical barrier for squirrels and raccoons. The extra wide, overhanging roof directs water away from the chamber, and gaps on the sides beneath it provide ventilation. Additional, small openings in the bottom corners drain any captured water, and the black tone of the roof contrasts the tan abode. A metal screen rests slightly above the wooden floor, preventing blowflies and other parasites from overtaking the nest, and the home is simple to place on a post, pole, or wall. One side panel lifts upward, exposing a clear plastic window beneath. This enables you to monitor the birds with minimal disturbance, and the plastic can be removed at the end of the season for thorough cleaning. The thick, cedar body remains durable for many nesting cycles, and resists damage from water and insects naturally. Watch the nesting activities of bluebirds from a distance with this Cedar Bluebird House with Viewing Window.
Opening: 1.5" dia.
Dimensions: 8"L x 7.5"W x 12"H
Mounting: pole or post mount