House wrens often compete for bluebird boxes

House wrens block the entrance to the box with twigs.  Photo by Ken Meyer

      My husband and I recently discovered that a house wren had taken over one of the bluebird boxes that we've been monitoring at a local park. We could tell that the nest had been built by house wrens because the twigs were much thicker than what bluebirds typically use.
      And before I pulled the nest out, I saw two tiny pink eggs on the bottom of the box. Both were broken. The pink eggs told us that this nest had been built by house wrens. 
      Although it's not visible in the above photo, there was a cozy cup lined with feathers in the middle of this mess. There were no eggs in the cup.
     House wrens sometimes fill the box to the top with twigs to create a barrier between cup-shaped interior and the entrance to the box, seemingly to protect it from cold weather and predators, including cowbirds.

     These tiny birds with the sweet song can be fierce competitors for natural holes and nest boxes. They're known to harass much larger birds, sometimes pecking the eggs and killing the young in a nest site they want to use. In some areas they are the main source of nest failure for bluebirds, tree swallows and chickadees.
      But before you go kicking them out of your boxes, remember that house wrens are native songbirds, which mean they are a protected species. So a completed nest and eggs should not be disturbed. However, it is perfectly legal to discourage a house wren from building a nest in a bluebird box by consistently removing the twigs it places inside.

     Want to learn more about bluebirds?  Go to >Southern California Bluebird Club.
     Happy Birding!
     J.J.