There are a few that tough it out and hang in there despite winter's snow, wind and cold temperatures, but bluebirds in large number remain a welcome sight each spring in Wisconsin.

Some of these winter warriors, I suspect, make quick 300-mile dashes south when weather turns, only to return a few days later.

Now is the time, however, to make sure those bluebirds that visit our offerings find these tiny cedar-sided motels fit for raising up to three families of fledglings this year.

While we don't provide heat for early morning cold snaps or air conditioning for a mid-July heat wave, there are things we birders can do to lessen the life-threatening weather extremes.

Cold, as well as heat, can kill young bluebirds crowded inside a nest box.

Accommodations are best built using thick cedar, the type that will insulate from cold and ventilate when the nest box temperature climbs above 100 degrees.

There have been debates over what constitutes the "best" bluebird house — ever since we started providing nest boxes to replace more natural accommodations such as cavities in wooden fence posts and tree limbs.

Leif Marking at the Brice Prairie Conservation Association near Holmen has generally been in the thick of these discussions — along with those in regards to wood duck houses.

Marking, a retired federal fisheries researcher, has led the way in the BPCA, conducting experiments to determine what works and what doesn't.

"To begin, we make all our bluebird houses using 7⁄8-inch cedar, not ?-inch material," Marking said. "This provides better insulation from cold and heat."

Marking is also a firm believer that these bluebird houses should be made from wood, not metal — as some are constructed — again for better insulation. On the other hand, metal works perfectly for wood duck houses, he said.

"Paint will destroy the breathing power of the wood. If a dark paint is used, the house will absorb more heat in summer, too. Cedar will last 15 to 20 years without paint, so why paint?" Marking said.

Marking uses data-loggers in bird houses to determine if shading materials might influence interior temperatures. Studies have determined that temperatures exceeding 107 inside a house are lethal to most baby bluebirds.

Considering temperature factors, along with early summer blackfly infestations, one of the best bluebird houses is one that keeps the cold out in May, keeps blackflies out in June and vents the interior in July and August.

One house, with a few slight modifications, can do it all, Marking said. He makes what he calls a convertible house, one that can be modified to have vents in summer and no vents in spring.

"In spring, the vents are closed, except for the oval opening where the birds enter. Then, in summer, the houses are vented, if necessary," he said.

Screws, not nails, are used to attach the two side panels to the back and front of each house. The left side panel is held in place and attached to the front and back with four screws, the right side with two screws, which allow the panel to be opened to clean and for inspection.

"In spring, the two side panels are up to the roof, with no opening for ventilation," Marking said. "This keeps the house warm and keeps the blackflies out."

If the weather gets hot during summer, for the second or third nesting, the screws holding the side panels can be removed, the panels slid down a half inch, creating a vent and the screws replaced at a lower point. Holes are pre-drilled for the two side panel settings.

Got questions? Visit the BPCA Web site at
www.briceprairieconservation.org.

Wisconsin loves bluebirds

More bluebird chicks (28,244) fledged out of nest boxes (7,800) in Wisconsin last spring than in any other state in the nation.

The number of chicks represents a national precedent, according to Kent Hall, a retired University of Wisconsin-Stevens Point biology professor and a vice president of the Bluebird Restoration Association of Wisconsin (BRAW) — the statewide cadre of volunteers who compiled the numbers.

Hall said the key to nest box success is "location, location, location," keeping them at least 100 yards apart to reduce competition for accommodations. BRAW research found that 90 percent of nest boxes can attract bluebirds if they are properly located.

Hall said bluebirds need a territorial forage area of 3 to 21 acres of predominantly open habitat. Ideal sites have short, sparse grass with interspersed trees, such as cemeteries, golf courses, parks, roadsides, and railroad tracks.

Hall said 850 BRAW members recorded an additional 10,000 tree swallows, 2,000 wrens and 400 chickadees that fledged from Wisconsin nest boxes in 2007, for a total of nearly 41,000 songbirds fledged.

Interested in bluebirds or nest box restoration? Contact Hall at 715-344-8081 or check out BRAW at www.braw.org.

— DNR, State Journal staff