I get a lot of questions about wren guards. House wrens are cavity nesters like chickadees and bluebirds. They are native birds, but very aggressive and territorial. They will build dummy nests in any available cavity they find, which is easily recognizable from the huge number of twigs they stuff in the space. The problem for other birds is that the wren will enter the nest of a chickadee or bluebird, and peck the eggs or even young nestlings. Unlike house sparrows, they can't kill the adults, and once the chicks are about 7-10 days old, the wrens can't harm them. Since they are native, they can't be managed in the same way as the invasive house sparrow. So, an easy and effective strategy is to put a wren guard on the nest box.

The purpose of a wren guard is to obscure the view of the entrance from the wren, while allowing the nest box occupant (bluebird, chickadee, etc.) to find it. I live in a suburb of the Twin Cities where there are plenty of house wrens, and I've had success with chickadee and bluebird nests in my back yard. A wren guard doesn't have to be complicated at all. I make mine with cardboard, duct tape, and a small metal bracket to hold the shape. See the photos below. A couple of very important things to remember:

• Put the wren guard on ASAP after the first egg is laid.
  • At that point the nest box occupant has bonded with the box, and will still find the entrance.
  • It might take her a few minutes to negotiate around the guard, but she will find it. Meanwhile, a passing wren won't.
  • Make sure you don't put the guard on before the first egg is laid! If it is installed too soon, your desired nest box occupants won't have bonded with the box yet, and will not be motivated to find their way around the guard.
  • If you wait too long to put the wren guard on, and there are wrens in your area, there's a good chance that the egg(s) will be destroyed.
• Take the wren guard off when the chicks are 7-10 days old.
  • At that point, the chicks are large enough that a wren can't harm them.
  • If the guard is left on beyond that time, it can interfere with feeding and fledging.

See more information about Discouraging House Wrens at sialis.org. Also, if you are having problems with house sparrows, try using a sparrow spooker. You can buy one or build one yourself.

Click on each photo to get details about that step. And be sure to take a look at my most recent modification, described in the last photo.