A water source for your bluebirds is very important here in San Diego County, California. A shallow birdbath will provide water for them to drink, as well as a place to frolic and bathe. Keep the water clean and full. To prevent algae, clean with either 1 part vinegar and 3 parts water or 1 part bleach and 9 parts water.
Bluebirds eat lots of grasshoppers, crickets, caterpillars, beetles, and worms. 82% of the Western Bluebird diet is comprised of insects and spiders. The other 18% comes from plant berries, especially during the winter. Some plant berries that bluebirds are attracted to are mistletoe, juniper, toyon, elderberry, oregon grape, hollyleaf cherry, coffeeberry, pyracantha, laurel sumac, lemonadeberry, currant, and palm fruit.
Bluebirds don't need you to feed them, as they know how to find wild food. However, a handout of food may help your backyard bluebirds early in the nesting season, when there might be a stretch of unusually wet or cold weather. You can also offer food to a bird that has lost its mate, to help it with the hard work of feeding a clutch of nestlings solo. Feeding bluebirds is also fun, and it brings the birds in close.
During nesting season, you can give your bluebirds a few mealworms (Tenebrio molitor) once or twice a day. You can buy mealworms dried or live, or raise your own. A dozen or so mealworms, offered each morning and evening in a bowl or feeder is enough for a pair of birds. And you can raise that to 50 to 100 at each offering when there are nestlings. Put the worms in the shade and where ants won't get them.
This well-made (and easy to clean) bluebird feeder was designed for Eastern Bluebirds, so the openings are a bit small for Western and Mountain Bluebirds. However, there is an easy way to widen the openings: With a pair of pliers, gently "pinch" every other opening on each row of mesh, being careful not to damage the coating on the mesh.
Non-native house sparrows can compete with bluebirds for nestboxes. House sparrows can ruin bluebird eggs and young, and may even peck an incubating female to death. House sparrows prefer small grain seeds, such as millet, cracked corn, and milo-- the stuff found in common birdseed mixes. If you like to feed birds, try offering nyjer (thistle) or safflower, as house sparrows, starlings and ground squirrels are not attracted to them. Place feeders away from nestboxes, and consider suspending feeding during nesting season to reduce attacks from Cooper's Hawks. Or a less expensive way to feed birds is to plant lots of seed and fruit bearing plants.