Bluebirds probably were never common, as they were limited by their nesting requirements. However, in the 1800s as colonists spread out across North America and cleared heavily wooded areas, bluebird habitat increased. The settlers also controlled prairie fires, which allowed more trees to mature and develop nesting cavities.
In the early 1900s, the bluebird’s future looked promising. But this era of good fortune was short-lived: more settlers arrived, land became more valuable, and many thousands of hectares of bluebird habitat were completely denuded of trees annually for farming. Also, Europeans brought with them intruders—starlings and House Sparrows—which had little difficulty in evicting the bluebirds from the few remaining nest sites.
Bluebird numbers continued to decline until some naturalists felt the bluebird was doomed to extinction. Fortunately, bird lovers across North America began in the 1920s to build bluebird nest boxes in an attempt to reverse the decline. This conservation effort really became popular during the 1950s and 1960s. The results have been encouraging.
Today, humans still compete for habitat with bluebirds. Forestry practices that remove dead trees and snags reduce the available nesting cavities for bluebirds. Some people have helped to offset this loss by creating trails of bluebird nest boxes. Unfortunately, sometimes other people vandalize nest boxes and destroy their contents.
Although bluebirds tend to arrive early enough in spring to get first choice of the available nest boxes, they must sometimes compete for them with Tree Swallows, House Wrens, Chickadees, House Sparrows, and European Starlings. Starlings can be excluded from the competition by an entrance hole no larger than 3.8 cm in diameter. In addition, each of the three bluebird species competes with the others for boxes, where ranges overlap.
Once installed, bluebirds are well able to defend boxes that are properly designed and placed to their liking. However, where House Sparrows are abundant they may enter a bluebird box and go so far as to kill young and adult bluebirds. Although it is too large to enter their nest boxes, the main avian predator of the bluebird is the American Kestrel, a small hawk. Domestic cats and raccoons are formidable predators of young and incubating female bluebirds. Deer mice and chipmunks can also be problems. Smooth metal posts will often prevent animals from climbing up to nest boxes. A parasitic blowfly Apaulina Stalia takes its toll of bluebirds in some areas, although it is not thought to reduce population size. The fly lays its eggs in the bird’s nest, and the larvae attach themselves to the young birds and may kill them by sucking their blood. This parasite can be controlled by dusting nest boxes with diatomaceous earth.
As a result of bluebird banding programs, we now know that some bluebirds nest when they are one year old. The age record for a Mountain Bluebird is four years and 10 months, and an Eastern Bluebird lived eight years. Banding has also shown that successful breeders often return to the same area or nest site each year.
Contrary to popular belief, very few young bluebirds return to nest inthe area in which they were raised. In one study area, less than one percent of the young fledged were found nesting in that area in subsequent years. Probably, many do not survive to breed, and survivors disperse to new areas. The erection of thousands of birdhouses by concerned individuals and organizations has been responsible for preventing further depletion of bluebird numbers and in many areas has increased populations. Bluebird nest boxes need not be fancy;
wood is the best material to use.
An ideal nest box for bluebirds would be a floor 20 cm square, walls 25 cm high, with a 3.8 cm diameter entrance hole located 18 cm above the floor.
The top of the birdhouse should be easily opened for cleaning. It should overhang the entrance hole to provide protection from rain. Two 6-mm holes should be drilled near the top of each side for ventilation and two 6-mm holes should be made in the floor for drainage. Perches should not be placed on the birdhouse, as they only encourage House Sparrows to take up residence.
If an exterior finish is desired to preserve the wood, a woodstain is adequate. If houses are painted, a light green, brown, or grey should be used. White is too conspicuous, and a white nest box is often shunned by bluebirds. Dark colors absorb heat, which may become too intense for the eggs or young in the nest box. Do not paint or varnish the inside of the box. The interior walls should have a rough surface to aid the bird in climbing up to the entrance hole.
The location of a bluebird house is important. Houses should be placed in semi-open areas such as pastures, fields, and rural roadsides. A fence post in a clearing with scattered trees about 20 m away is probably a good location. Bird houses in urban areas or near farm buildings are usually occupied by house sparrows.
The nest box should be placed on a post 1 m or more above the ground. It does not matter which direction the front of the house faces.
A “bluebird trail” consists of several nest boxes spaced 200 m or more apart and in a manner convenient for inspection on foot or by car, to record nesting success, band young birds, and clean out boxes in the fall. Regular inspections also allow the nest caretaker to remove nests of House Sparrows and other intruders. Some trails may be only a kilometre long, while the longest runs several hundred kilometres from Winnipeg, Manitoba, to North Battleford, Saskatchewan, with hundreds of kilometres of branch lines leading off from the main trail.
——— reprinted with permission from Environment Canada, “Hinterland Who’s Who”.