The North American Bluebird Society's general guideline for the distance between western bluebird houses is about 150 yards, 125 to 150 yards for eastern bluebirds, and 200 to 300 yards for mountain bluebirds. You can try putting the houses closer together if there is something between them that would prevent the birds from seeing each other, such as a building.
How Far Apart Should Bluebird Houses Be?
Evenly Spacing Bluebird Houses
Pairing Bluebird Houses
To prevent competition for bluebird houses, the houses can be paired. To do this, place two houses 5 to 25 feet apart from each other. The bluebird society's spacing guidelines could then be used to determine the distance between that pair and the next. This allows a different species to occupy one of the paired houses while a bluebird claims the other.
Although bluebird houses can be spaced an equal distance apart, the better option is to set up your bluebird houses in pairs. Placing two houses close to each other gives bluebirds a good chance of claiming a house and raising a brood without another species taking it first or even puncturing the bluebirds' eggs and demolishing their nest.