The Eastern Bluebird has many competitors. House Sparrows, Starlings, Wrens, Chickadees, Swallows, etc. Most of these competitors get along with Eastern Bluebirds and should be left alone to nest, however a few get along terribly with them and should be deterred from entering a potential Eastern Bluebird nesting site.
Chickadees, swallows, and martins should be left alone. They will not harm Eastern Bluebirds, and Tree Swallows have even been known to protect them from predators if they are nesting close to each other. House Sparrows and Starlings should be removed and killed or relocated (refer to Predators). They are known to harm and kill Eastern Bluebirds. The only thing you can do (legally) about House Wrens is remove their dummy nests (PLEASE! Do not let House Wrens use your Eastern Bluebird boxes!). You can not legally remove eggs, fledglings, or adult Wrens without a license. For more info e-mail me.