If you're queasy about violent behavior, even among birds, or if you're looking for variety in the bird life in your yard, you might want to give the wrens a second thought. You see, despite some authors referring to wrens as inoffensive, they do have a nasty streak. I'm talking about egg-pecking. No, not their own eggs. Other birds' eggs.
Wrens are famous for it.
House, marsh, sedge, Bewick's, cactus, and probably if you looked hard enough, just about every other kind of wren in the world does it. Cowbirds, of course, do not hatch out their own young, but instead lay their eggs in other birds' nests. They usually pick on certain species that are unable to recognize the strange eggs and get saddled with raising the cowbird young. Wrens, on the other hand, have slightly different motives for egg-pecking. By destroying the eggs of other species nesting in the vicinity of their territory, the wrens reduce competition for limited resources.