This article has been adapted from the Audubon Birdhouse Book by Margaret A. Barker and Elissa Wolfson. You can also download a printable PDF of the instructions here.
In the natural world, all three species of North American bluebirds—the Eastern, Western, and Mountain Bluebirds—seek tree cavities or woodpecker holes for nesting sites. But today, natural cavities can be hard to find. Competition for these limited sites is a huge problem, especially in early spring. Old and rotting trees often are removed. Not long ago, many bluebirds nested in wooden fence posts, especially around farms. Many of those have been removed or replaced with treated wood, plastic, or metal posts. A well-built and well-placed blue-bird nest box in your own backyard or nearby park can help boost local populations.
Even with nest boxes in place, bluebirds must compete with both introduced and native species that also want to call these nest boxes their home. Knowing where to place and where not to place bluebird nest boxes is critical. Chickadees and titmice, for example, prefer nest boxes near or under mature trees or within woodlands and forests. By contrast, bluebirds like nest boxes out in the open; even a small yard with open spaces will suit a bluebird. Most important, bluebirds need to live near a ready supply of insect food.
Since bluebirds defend large feeding territories around their nests—one or two acres in early spring—they don’t want to nest close to other bluebirds. Tree Swallow pairs won’t nest close to one another either. So reduce competition by installing pairs of bluebird nest boxes no more than fifteen to twenty feet apart. Bluebirds may nest in one, and swallows, chickadees, or titmice in the other. This “peaceable kingdom” occurs for practical reasons: These bird neighbors, by and large, do not share the same food supply. But the nest box “pairing” idea is not with-out differing opinions. Some people believe it encourages other species more than it accommodates bluebirds. Others put up a second box nearby only when a non-bluebird species has claimed a nest box first.
Ironically, the cutting of Eastern forests, especially pine woods, for agriculture in the nineteenth century may have actually benefited Eastern Bluebirds by creating additional foraging and nesting habitat. More recently, however, their populations have been affected by loss of habitat and tree cavities, unusually cold winters in the 1960s and 1970s, egg and chick predation, and competition for nesting sites by introduced House Sparrows and European Starlings.
Eastern Bluebird Basics
Range: Eastern Bluebirds are year-round residents in the southern United States. They typically begin nesting as early as January in the south and in March in the northern United States and southern Canada. They are also found in parts of Mexico, Central America, and Bermuda.
Field marks: Males are bright blue above and rusty orange below with a white belly. Females look similar but have a blue-gray back and lighter orange underparts. Both sexes are seven inches long and stout billed. In flight, look for short blue wings and tail.
Voice: The call is a musical chur-wi or tru-ly. The song is a series of three or four soft musical notes, often described as a warble.
Feeding: Keen-eyed Eastern Bluebirds feed on ground-dwelling insects, including beetles, crickets, grasshoppers, and caterpillars, which they can spot while perched as far as 150 feet away. They pounce upon their insect prey, then fly to a perch and strike it against a hard surface before feeding—or they just catch and eat insects in the air. Adults also feed juicy, high-protein spiders to nestlings. Wild berries are also eaten, especially in colder months. At feeders, offer raisins, currants, suet mixes, and mealworms (live or freeze-dried).
Western Bluebird Basics
Range: Western Bluebirds are found in southwestern Canada, Mexico, and many western U.S. states. They are medium- to short-distance migrants that winter in the southern part of their range and begin nesting in early April.
Field Marks: Adult males have cobalt blue wings and tails as well as an all-blue head, chin, and throat, and a white belly. The upper breast is chestnut with varying pat-terns of blue and gray. The back may be partly or entirely chestnut. Females are a paler, grayer version of the male.
Voice: The song is a series of call notes described as few or kew. Chatter calls sound like cut-cut-cut. Soft tch-tch-tch calls can also be heard.
Feeding: Western Bluebirds eat insects in warm weather, and fruits and berries in winter. Mistletoe and juniper berries are favorites, and they love mealworms at feeders. They are often seen “fly catching” or foraging on the ground, using low branches as a jumping-off place.
Mountain Bluebird Basics
Range: Mountain Bluebirds are found primarily in the western mountains from east-central Alaska to south-central Mexico, migrating to the northern parts of their range to begin nesting in late April.
Field Marks: Breeding males have a turquoise-blue back, a paler blue breast, and white belly and under tail coverts. Females and juveniles are gray above and have pale blue wings and tail and a buffy chest. The adults are slightly larger and thinner-billed than other bluebirds; their wings are proportionately longer than the other bluebird species.
Voice: The Mountain Bluebird’s call is a low fewor chur, described as a soft burry chortle. The male’s short, subdued warbling song has been called “hauntingly beautiful.”
Feeding: Mountain Bluebirds feed on insects, including weevils, wasps, beetles, bees, grasshoppers, caterpillars, and crickets. They often “hover hunt” like kestrels when forag-ing, or hunt from low perches before dropping or darting upward to capture prey. In late winter, they rely on native berries of mistletoe, hackberries, juniper, and hollies.
In southern states, bluebirds may start looking for nesting sites as early as January, so have nest boxes ready for them. However, this may be two to three months later at their northern limit. During the breeding season, check nests at least once a week. Since bluebirds typically lay eggs in the morning, the ideal time to check nests is in the afternoon. Include perches in your bluebird landscape. Both adult and newly fledged bluebirds like to sit on small trees or fence posts from which they can scout for insects on the ground. Bluebirds are mostly insectivorous, but they also eat wild berries. Offering mealworms near nesting sites, and planting berry-producing grapes, blackberries, dogwood, elderberries, and serviceberries, might induce bluebirds to stay around your property. Supply fresh water for both drinking and bathing. Many “bluebirders” remove old nesting material from a nest box right after the young have fledged; this task can be repeated several times during the nesting season. At the end of the nesting season, clean out the nest box one last time and make any needed repairs. Bluebirds and other species often use nest boxes for roosting in cold weather.
Nesting habits: Female bluebirds build tight cup nests atop a looser built base. Thin bark strips, pine needles, and dry grasses are typical nesting materials. The inner nest cup may be lined with softer, finer materials.
Eggs: Range from two to seven pale blue and, very seldom, white eggs. First clutches average five to six eggs; second clutches average four to five eggs. Eastern Bluebirds typically have two clutches a year, but in warmer climates, three clutches are common. In the northern part of their range, Mountain Bluebirds are known to lay larger but fewer clutches than Eastern or Western Bluebirds.
Egg-laying: Typically one egg each day until the clutch is complete.
Incubation: Female incubates for twelve to fifteen days. Male feeds incubating and brooding female.
Days to fledge: Fledge dates for Eastern and Western Bluebirds may vary from sixteen to nineteen days. Mountain Bluebirds typically fledge within 17 to 21 e days.
In the 1960s, self-taught naturalist Dick Peterson noticed the decline of local bluebirds. As a way to help them, he designed a wooden bluebird box to replace their preferred but scarce natural tree holes. His unique “Peterson” nest box, with its signature sloping roof to thwart predators, is credited with helping restore bluebird populations in Minnesota and elsewhere. In the late 1970s, Peterson received an outpouring of letters and requests for nest box plans following a widely read Minneapolis Star Tribunearticle on his bluebird work. Inspired by this surge of interest, in 1979 Peterson partnered with the National Audubon Society’s Minneapolis chapter and formed the Bluebird Recovery Program (BBRP) of Minnesota—the nation’s first state bluebird organization. Keith Radel, a BBRP coordinator who knew Peterson, says his legacy extends beyond designing a nest box and founding the organization: “Dick’s real influence was teaching people responsible ways to keep birds safe—how to identify and then fix problems at the nest box. His insistence on weekly nest checks has been critical to bluebird recovery.”
• Lumber: cypress (used here), white cedar, hemlock, or local weather-resistant wood with low toxicity
• One 1x10x11” (roof)
• Four 1x6x10” (front, sides, and back)
• Two 1x6x4” (floor and inner roof)
• One 2x2x9” (pole-mounting block)
• Exterior screws: twelve 1 5/8” (basic construction); two to six 1 1/4” (roof to inner roof); and two 2” (pole mounting block to back)
• Caulk or sealant (sealing between top and inner roof)
• One 2 1/2” galvanized nail (bent, latch nail)
• Mounting: One 1/2”x5’ galvanized metal conduit, one 1/2”x4’–5’ steel rebar (for stake), and one conduit coupler
1. Hole saws were used for the xbox entrance and ventilation holes, as well as the mounting block. A table saw with its blade lowered was used for the drip kerfs on the underside of the roof and for the ladder kerfs on the inside of the front.
2. The back piece of the xbox is attached to the inner roof. Two deck screws (1 5/8”) are installed with an impact driver.
3. Test-fit the attached back, unattached sides and inner roof. Use a pencil to mark the placement of the recessed floor. Drive in screws.
4. Top of sides are attached to the inner roof above the entry hole.
5. Pivot screws, driven into the front piece from the bottom of both sides, allow the front to open easily for checking and cleaning.
6. One galvanized nail (2 1/2”) is bent to create the latch nail. Drill the latch nail hole slightly downward.
7. The mounting block for the gilbertson pole system is installed on the back of the xbox with two exterior deck screws (2”). Note the predrilled 3/4” hole on the mounting block.
8. Apply a bead (line) of all-purpose low Voc caulk to the top of the inner roof prior to installing the exterior roof.
9. The gilbertson pole system is easy to assemble. Drive rebar into the ground, leaving two feet above ground. Attach conduit coupler to end of conduit. Tighten upper, shorter screw against conduit. Slip coupler over rebar. Tighten lower, longer screw against rebar. Clean pole with steel wool and coat it with furniture polish. Add baffle if needed.
Mounting: The Xbox is designed to be mounted onto a half-inch conduit/rebar pole, called the “Gilbertson system”. Avoid mounting nest boxes on fences or trees where climbing mammals or snakes are present. Use predator guards to further block nest box access.
Height: Bluebirds nest within a wide range of heights, from two to 50 feet. Mounting at eye level provides easy checking; however, if cats or other predators are problems, hang nest boxes at least six to eight feet from the ground.