Cavity Nesters

Bird houses have been readily accepted by many natural cavity nesters, and increases in breeding density have resulted from providing such structures...


Bluebirds are secondary cavity nesters, which means they need a hole or cavity that something else has excavated. In nature it is often the abandoned woodpecker cavity that the bluebird uses to nest in. Bluebirds and woodpecker population trends go hand in hand. When natural cavities are lacking, humans can provide a nestbox to encourage them to nest.

Mountain and Western bluebirds prefer open spaces with scattered trees and low sparse vegetation. They can be found in open farmland, orchards, forest edges, golf courses, cemeteries and desert-like areas. Successful nesting is based on putting the nestbox in the right habitat.

Bluebirds prefer a certain size and dimension of nest cavity. The opening to get into the box or cavity is also important. The Johnson slot nestbox accommodates all three species of bluebirds, which includes Eastern, Western and the Mountain Bluebird. It has been tested and approved for this use by the North American Bluebird Society. The slot opening is one and three sixteenths inches. It has a saw kerf under the roof to direct water away from the entrance and has plenty of ventilation for warm environments.

Boxes are mounted at eye level for easy monitoring. They can be mounted on fences or trees, however it is ideal to have a dedicated pole for each box. Mount them so they face away from prevailing winds and are shaded from hot afternoon sun. When the young make their first flight (fledge) to safety, they fly about 100 feet so face the box toward a tree, shrub or post in the distance. Never put boxes on utility poles or on private property unless you have obtained permission from the owner.

If you have predators such as raccoons, snakes, cats, chipmunks, squirrels or bears you may have to use special precautions. It is generally recommended not to put up nestboxes in an area where there are cats because bluebirds feed by catching insects on the ground and may be easy prey for felines.

Predator guards can be used on poles below the nestbox or on entrances to protect bluebirds.

Spacing of boxes - Spacing of about 300 to 400 feet apart is about right to provide enough area for the bluebird to search for food in. However if swallows use the boxes, pairing the boxes 15 to 20 feet apart may allow the bluebird and the swallow to nest side by side. One theory suggests that a tree or violet green swallow will evict a bluebird unless there are two boxes available. There do not seem to be any theories regarding other cavity nesters such as nuthatches and chickadees and their relationship with bluebirds. Recently one person noted that an American Kestrel and bluebird family nested successfully in boxes on the same post. You have to be willing to experiment a little by adding and deleting boxes depending on the situation.

Since the Johnson Slot box has a roof overhang on all sides, a piece of 2X4 (7inches long, with one end cut on an angle) is nailed to the tree or fencepost first (with the sloping edge away from the box) using two 4 inch box nails. The nestbox is then attached to the 2x4 with two #12 21/2-inch wood screws, leaving the top back air vent clear of the 2x4. This prevents the box from sloping forward and keeps the floor level.

The finishing touch (where available) includes putting a handful of preservative-free coarse wood shavings in the box - making it more like a natural tree cavity.

Want to make your own nestbox? For a copy of the Johnson Slot Nestbox plans, click here.