My colleagues and I often hear concerns that wrens have “taken over” their bluebird nest boxes. House wrens are small, brown birds that are native to North America. They are migratory and are thus protected by the Migratory Bird Treaty Act.
Wrens can sometimes cause problems for bluebirds because the males build “dummy nests” in multiple places to give their mates multiple options for nesting sites. This limits the availability of nesting cavities for bluebirds. House wrens have also been known to enter the nests of other birds and poke holes in the eggs, or remove an entire nest from a nest box.
If you have problematic house wrens, remember that by law, you cannot harm wrens or disturb their completed nests (a completed nest will have a nest cup) or eggs. It’s best to prevent problems before they arise, so you can try moving nest boxes into more open habitat, at least 50 feet away from the brushy and heavily wooded areas wrens favor. You can also install wren guards after bluebirds have laid their first egg, and remove incomplete or dummy nests to try to discourage wrens from using the nest box. If you are uncertain if a nest is incomplete or a dummy nest, err on the safe side and leave it alone until it’s time to clean out nest boxes at the end of the summer.
But consider also the benefits of house wrens. They are insectivorous, so they help to control insect populations. Their energetic movements – hopping amongst brambles and low branches, occasionally stopping to sing – provide great opportunities for bird watching. In fact, the males sing a lovely, bubbly song as they search for a mate and establish a territory.
For more information about wren and nest boxes, I recommend the Sialis.org website. You can also call or email your WHC Regional Biologist for advice.