More Cavities Needed
Bluebirds live in cavities, but they can't make their own cavities. Like woodpeckers, bluebirds prefer trees in open areas. The best habitat for bluebirds is an open area with scattered trees such as an old garden. By clearing sections of forest to create more fields, early settlers happened to improve bluebird habitat. Bluebirds feed on insects on the ground, and insects were abundant in the fields.
In the early 1900s, eastern bluebirds were one of the most common songbirds, but by 1969 fewer than 100 bluebirds nested in Minnesota. Other states also reported severe declines in the populations of all three bluebird species.
Why did the populations decline? One factor may have been the widespread use of insecticides, but the most important factor was probably the loss of habitat. At first, fences were built with wooden posts, and wooden posts develop cavities. While bluebirds like fence posts with cavities, farmers do not. Cavities make the fence posts weak, and cattle escape through weak fences. After World War II, most farmers replaced wooden posts with new posts made of steel.
With less suitable habitat, the bluebirds were forced to compete with other birds for the few remaining nesting sites. More aggressive birds such as the tree swallow and the house wren often drive the bluebirds from their nests. Starlings and house sparrows — both introduced to this country from Europe — also compete with the bluebirds for nesting sites.
All three bluebird populations — eastern, western, and mountain — declined nearly 90 percent between 1935 and 1985. The North American Bluebird Society and others are working to increase the number of bluebirds. The most successful method is to improve the habitat by providing the bluebirds with boxes as living places. Thousands of boxes are placed and being managed by individuals interested in helping the little bird.
If placed in the right environment, the boxes do attract bluebirds. Bluebirds prefer boxes placed next to open areas. If placed near a brushy area, the boxes are more likely to attract house wrens. Tree swallows would compete for isolated sites in open areas. Competition from tree swallows will decrease if the boxes are placed in pairs, less than three feet apart. Boxes for mountain and western bluebirds must have slightly larger holes than those for eastern bluebirds, but if the holes are too large, starlings will take the boxes.
Another cavity programme helped the wood duck population. When seeking for houses, the female wood duck looks for a cavity in a tree that is in or near the water. Often she chooses a cavity with a hole too small for a raccoon to enter. Human activities — logging mature trees and hunting — threaten the wood duck population. In 1918, the United States and Canadian governments banned the hunting of this species. Groups of citizens and conservation organisations built living boxes and placed them on poles or in trees that are in or near the water. Today the wood duck population is greater than one million, and hunting is once again permitted.