Bluebirds and Open-topped Nest Boxes

Bluebirds and Open-topped Nest Boxes
For years many bluebird enthusiasts have attempted to minimize the chance that House Wrens will occupy nest boxes intended for bluebirds. Although people enjoy the bubbly song of wrens, this species also has the habit of usurping bluebird nests. Tell-tale signs of this are finding bluebird eggs on the ground with a small hole, from a wren's bill, in each egg, and sticks on top of the bluebird nest.

Tests of a variety of nest box styles have indicated that wrens appear to be less discriminating than bluebirds and swallows, and will use nearly any style of nest box. The House Wren's small size eliminates the possibility of exclusion with small hole sizes.

To date, the best recommendation for avoiding wren use of nest boxes has been to move nest boxes as far as possible from brushy habitat, keeping them in very open spaces. Discussions at many North American Bluebird Society meetings have confirmed that wrens are moving progressively greater distances from brush to nest. It may be that the preferred brushy habitats are all occupied, and less, dominant birds are moving into poorer quality (more open) habitats.

Open-topped nest boxes have been used successfully by Vince Bauldrey in Wisconsin for well over two decades, and it has been suggested that they may deter House Sparrows and House Wrens. Many bluebird enthusiasts have been reluctant to endorse a box with the top directly exposing occupants to precipitation. B. Orthwein in Ohio has suggested adding a raised roof above an open-topped box. This modification would give the advantage of the well-illuminated box which may discourage some species' use of the nest box while still providing protection from precipitation.

Open topped, raised roofed nest boxes do indeed deter use by House Wrens. Nine test boxes were standard nest boxes with 4 inch by 4 1/2 inch floors. The box tops were removed and replaced with 1/2-inch hardware cloth. A solid roof was placed 4 inches over the wire roof. Individual nest boxes were placed at sites where House Wrens had nested in recent years. A thin board with a 1 1/8-inch diameter entry hole was placed over the original 1 1/2-inch diameter hole to exclude bluebirds. This was done to ensure that bluebirds would not be attracted to the sites to nest, only to have their nests destroyed by wrens.

Between four and six experimental boxes were used by wrens each year. Black-capped Chickadees nested in one of the boxes during two years and in one box during another year. overall, wrens used six of the nine boxes at least one year.

It does not appear that open-topped raised roofed nest boxes provide a significant deterrent to House Wrens. Proper location of nest boxes remains the best wren deterrent. If individuals do not want wrens using their nest boxes, they should move boxes further from brushy sites, or if necessary remove any boxes from sites persistently used by wrens. Moving nest boxes from sites of historic wren use to new sites, almost completely eliminates wren predation of bluebird nests.