The mountain bluebird is a medium-sized bird weighing about 30 g with a length from 16–20 cm. They have light underbellies and black eyes. Adult males have thin bills and are bright turquoise-blue and somewhat lighter underneath. Adult females have duller blue wings and tail, grey breast, grey crown, throat and back. In fresh fall plumage, the female's throat and breast are tinged with red-orange, brownish near the flank contrasting with white tail underparts. Their call is a thin 'few'; while their song is warbled high 'chur chur'. It is the state bird of Idaho and Nevada. It is an omnivore and it can live 6 to 10 years in the wild. It eats spiders, grasshoppers, flies and other insects, and small fruits. The mountain bluebird is a relative of the eastern and western bluebirds.
The Mountain Bluebird, Western Bluebird, and Eastern Bluebird
all prefer the same house dimensions:
Hole size 1 1/2"
Hole height above floor: 6" - 7"
Interior size: 5" x 5"
Why Bluebirds Need Houses
All bluebirds are cavity-nesting species, and they need safe, secure locations to raise their broods. Unfortunately, they are not assertive, and more aggressive species can easily drive bluebirds out of prime nesting spaces.
European starlings and house sparrows, both invasive species, will easily usurp nesting cavities, evicting and even injuring or killing bluebirds in the process. Bluebirds may also be subject to brood parasitism from brown-headed cowbirds, and young cowbird chicks can smother bluebird hatchlings and keep them from getting sufficient food and care. Continuing human development, particularly in the eastern bluebird's range, has removed many natural cavities these birds need for successful nests, making proper birdhouses even more critical.
Bluebird House Dimensions
Choosing the right sizes for a bluebird house is essential to ensure that not only are the birds safe and comfortable, but that other species are less able to use the house and the bluebirds will not be molested.
The entrance size is the most critical dimension for a safe, effective bluebird house. The hole should be 1 1/2 inches (3.8 cm) in diameter for eastern and western bluebirds, though mountain bluebirds occasionally prefer slightly larger holes that are 1 9/16 inches (4.0 cm) in diameter. Over time, holes gradually enlarge as edges are worn away by talons, inquisitive hatchlings, and general wear. It is wise to regularly repair the entrance hole with a sturdy block or plate to keep it at the appropriate size and keep larger, unwanted birds from investigating.
The entrance hole should be 6-10 inches (15-25 cm) above the floor of the birdhouse. This ensures that growing hatchlings are not able to tumble out of the opening. This distance also provides enough space that predators cannot easily reach hatchlings or brooding adults from the outside. Adding rough scoring or a piece of mesh below the hole inside the house can help birds reach the entrance when they wish to exit, but it is not strictly necessary, as their talons are ideal for climbing inside the house.
Interior Floor Space
Bluebirds raise 3-8 chicks in each brood, and a large brood of growing hatchlings can quickly become crowded if the house is too small. Ideally, the interior floor space of a bluebird house should measure 5x5 inches (12.7x12.7 cm) to accommodate the entire brood snugly but without too much excess space that can chill hatchlings.
Total House Height
The total height of a bluebird house can vary from 8-12 inches (20-30 cm). The rear of the house is usually slightly taller than the front, and the roof slopes down from the back to the front to provide cover and shade over the entrance. A smaller house is too easy for predators to access, and a taller house can be too difficult for young birds to safely exit when they are ready to leave the nest.
It is easy to use the proper dimensions when building a birdhouse, but birders who prefer to purchase birdhouses should carefully measure first to be sure the size and proportions are ideal for bluebirds.
Bluebird House Placement
Even a perfectly sized house will be of little use if bluebirds don't like where it is positioned. Proper habitat is critical, and open woodlands and forest edges along golf courses, in parks, near pastures, or adjacent to farmlands are ideal places to encourage nesting bluebirds.
They will nest in backyard houses as well, provided there is sufficient open space for foraging and nearby perches to use.
A bluebird house should be mounted on a pole or post roughly 4-6 feet (1.2-1.8 m) above the ground in a relatively open area, with the entrance facing a large tree or shrub 25-100 feet (7.6-30.4 m) away. That vegetation will provide convenient foraging for adults and it is a safe and reachable escape for fledglings when they first leave the nest. If more than one house is to be used, they should be positioned roughly 100 yards (92 m) apart to give birds adequate space to feel secure. Baffles are essential on poles or posts to deter birdhouse predators and further protect both the adults and nestlings.
Because bluebirds will use birdhouses as winter roosting spots, there is no best time to put out houses for them–anytime is good.