Predator GuardsWren Guards by Bob Orthwein

Wren Guards by Bob Orthwein

1996 Experience with Experimental House Wren Guards

by Bob Orthwein
(retrieved from defunct

Three experimental house wren guards were attached to nest boxes occupied by chickadees during the 1996 season. The wren guard is based on the idea that when wrens can’t clearly see the entrance hole, they will not evict nesting chickadees. The guard is a thin piece of wood that extends from the front edge of the roof to just below the entrance hole. For chickadees I used a space of 11/4″ between the guard and the entrance hole.

One of the chickadee nests was in my backyard in the city where wrens destroyed a nest in 1995. The 1996 nest was 25 feet from my house and I was able to observe the chickadees from nest building to fledging.

The nest was in a light green 4″ x 4″ box with the 19/16″ (40mm) hole reduced to 11/8″ using a polycarbonate (GE Lexan) plastic restrictor. The tough plastic restrictor keeps house sparrows out and prevents woodpecker damage. Even though house sparrows can not enter the 11 /8″ entrance hole, these weird birds will often mercilessly harass nesting chickadees by hanging on the box, poking their heads in the entrance hole and attacking the chickadees entering and leaving the box. The sparrows will do this even with a nearby empty box that they can use. A wren guard stops this harassment.

As soon as incubation started, I attached the wren guard to the backyard box. When the female chickadee returned to the box, she took 8 minutes to adjust to the guard and enter the box. The male took almost 4 hours to enter the box.

Incubation went on in a normal manner and the male dutifully maneuvered behind the guard to feed his mate on the nest.

Only 4 of 6 eggs hatched and I suspect that the 2 infertile eggs may have been chilled by several very cold nights before incubation started.

Feeding and brooding of the young went on normally and 4 healthy chickadees fledged with the guard still on the box.

A singing male house wren was heard in the area when the young were still small and vulnerable but no damage was done. After the chickadees fleged, the wren guard was left on and the box was cleaned out, but no wren sticks appeared. Wrens eventually built a nest in a box about 50 yards away.

Ohio Bluebird Society (OBS) member Jeff Davis attached a wren guard to a box when chickadee egg laying was in progress. Six chickadees fledged from this box in 1996. This same box, also with a guard attached, fledged 7 young in 1995. After the chickadees fledged, the guard was removed and the wrens had a nest and eggs within a week.

In late spring I had a chickadee add a little moss to a box and lay 2 eggs on the bare wood floor. Apparently, on this late nesting attempt, the chickadees just ran out of hormones and abandoned the meager nest. I put a wren guard on the abandoned box to see what would happen to the unguarded eggs. The 2 eggs were still there in September after the wrens went south.

In 1996 OBS member Darlene Sillick fledged 6 chickadees from a wren guard equipped box and OBS member Dick Tuttle fledged 7 titmice. Dick used a 2″ space between the guard and the entrance hole for the titmice. None of us used guards on any boxes with bluebird nests.

I plan to experiment with a few changes on some of my guards for the 1997 season. My back yard observations indicate that the chickadees entered the box by flying under the guard and up to the entrance hole. They did not go through the open sides. I plan to add thin pieces of plastic to the sides of the guard. The plastic will let in light but will be dull enough to obscure the entrance hole. I will increase the space between the guard and the entrance hole from 11/4″ to 11/2″ for chickadees.

All of my chickadee boxes are on steel posts. Don’t make mouse boxes or raccoon feeders by mounting boxes on wooden posts or trees. If you don’t use raccoon baffles, apply grease to the middle third of the post.

I attach the wren guards with screws and paint them the same color as the box. I try to put my chickadee boxes in the woods when possible whereas I try to locate bluebird boxes 40 yards in the open away from brushy areas. I hope to try wren guards on some bluebird boxes soon. I feel the bluebird guards should have at least a 2″ space between the guards and the entrance hole.

If further research finds that bluebirds will accept the guards, it is possible that house sparrows could be deterred from evicting and killing nesting bluebirds. Used in conjunction with triple boxing and house sparrow control, the guards could become an important bluebird management tool. Over management perhaps, but “Natures Way” vanished when the house sparrow arrived.

Before I attach guards to bluebird boxes I want to be sure I have a proper location and the time to observe the reaction of the birds.

I am fairly comfortable attaching wren guards to chickadee boxes even before a full clutch of eggs is completed. Much further testing and observation is needed to determine the effectiveness and best use of the guards, particularly on species other then chickadees.

Knowing how persistent, agile and aggressive house wrens are, I have been surprised and encouraged by the success of wren guards.

Reprinted from Ohio Bluebird Society’s Bluebird Monitor, Winter 1996, Vol. 14, No.8. n

Bob sent in his article above and noted: “To date, none of us in OBS that have used the guards on chickadees for the past two years have had any of them abandon their nests. Of course, the old “40 yards in the open away from wren habitat” rule should be used whenever possible for bluebirds.”

House Wren Nesting Timetable: See House Wren Biology


Pribil S., date? An Effective Trap for the House Wren, North American Bird Bander, Vol.22, No.1, p. 6 – 9

Knowing how persistent, agile and aggressive house wrens are, I have been surprised and encouraged by the success of wren guards.
– Bob Orthwein


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