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April 2001 Thinking out of the box A new nest box design borne of careful observation boosted bluebird populations statewide. Joe O'Halloran The female bluebird takes a peek inside while the male stands guard. © Jack Bartholmai

In the early 1900s, populations of the little bird that "carries the sky on its back" began to dwindle in the United States and Canada. Loss of habitat and nesting space, exposure to pesticides, and competition from European starlings, house sparrows and other non-native species had so diminished bluebird populations that the species needed human assistance to improve its chances for survival.

Fast-forward to the 1970s. Thousands of volunteers participating in a grassroots bluebird conservation movement began putting up nest boxes in hopes of strengthening the songbird's numbers. The effort continued throughout the ensuing decades.

Bluebirds responded with equal zest. Today, populations of the bird Henry David Thoreau said "reflected the sky itself" are thriving.

To the delight of bluebird monitors in Wisconsin, the number of bluebird babies reported flying out of nest boxes around the state recently doubled – from about 5,000 bluebirds in 1996 to about 10,000, in both 1998 and 1999. Curiously, the increase resulted from having fewer, not more, nest boxes.

The Bluebird Restoration Association of Wisconsin (BRAW), a nonprofit organization dedicated to the bluebird's revival, believes the increase occurred due to some out-of-the-box thinking. When almost all of Wisconsin's volunteer bluebird monitors switched over to a new style of nest box placed on a predator-proof post in a different configuration than what had been used in the past, the bluebird population blossomed.

Old nest boxes: Too deep and too many

Once the most widely used box in Wisconsin, the Hill Lake nest box was a "deep box," sometimes with 9 to 12 inches from hole to floor. The depth was supposed to put the nest cup out of the reach of raccoons, but some bluebirds defeated this purpose by building very tall nests. Sometimes the nests went all the way up to the box opening, offering an easily accessible snack of eggs, nestlings and adult birds to hungry raccoons.

Although the Hill Lake was the most commonly used nest box, BRAW members were also testing different nest box styles and hole shapes to find the most effective means to attract bluebirds and deter competitors. Nest-box monitors collected data on about 15 different box designs. From their data and observations, the BRAW monitors learned bluebirds preferred to nest in boxes with a smaller volume -- the smaller the better. If tree swallows were around, bluebirds used very few of the deep boxes (in some areas, less than 10 percent).

A second part of the old method was to place nest boxes close to one another in pairs. The idea was that tree swallows would commandeer one box, leaving the other free for bluebirds. As it turned out, "box pairing" produced few bluebirds, but plenty of tree swallows and wrens.

Drs. Linda Wittingham and Peter Dunn, ornithologists at the University of Wisconsin-Milwaukee, understood the "why" behind the box-pairing data. The aggression of polygamous male birds, such as tree swallows and wrens, was heightened at paired box sites. The males tended to "protect" or reserve an extra nesting box within about 100 feet of their first nest, sometimes installing a second, simultaneous mate.

In 1996, at BRAW's request, Wisconsin monitors began reporting paired-box data separately from single-box data. The numbers immediately showed tree swallow production was much higher in paired boxes. Furthermore, 100 paired boxes produced little more than half the bluebirds produced from 100 widely spaced single boxes.

BRAW also discovered some very efficient craftspeople were crowding their sites with too many nest boxes, resulting in larger numbers of tree swallows, and ever fewer, sometimes zero, bluebirds. Surprisingly, adding more bluebird boxes in a limited area decreased, not increased, the number of bluebirds produced.

Regardless of the distance between paired boxes, the single boxes out-produced them again and again. The switch away from deep, paired boxes began. Five seasons and data from 20,000 boxes have convinced us that the new approach is the one to take.

New nest boxes: Shallower, safer, and single

Almost all BRAW monitors now use small volume nest boxes, mounted on raccoon-proof posts and positioned as widely spaced singles. Typically, the small-volume boxes have only about 3 1/2 to 5 inches from hole to floor. The "Peterson style" is the box most commonly used. On first impression, this box appears to be quite large. But of all the standard boxes, it has the smallest nest volume. The boxes work best spaced at least 100 yards apart – the early, traditional way of bluebird box spacing.

We protect nest boxes from pillaging raccoons and squirrels with what I call a "three-minute post" made from rebar and conduit pipe. I have about 40 nesting boxes on such posts, and for the past five years have experienced no raccoon predation. Unlike wooden posts, these posts show no sign of deterioration. Last year, those 40 boxes fledged 154 bluebirds.

The modern box has a shallow hole-to-floor height. © Joe O'Halloran

Correct placement is important. If you put a box within a raccoon's reach from a fence, tree or other climbable structures or objects, even the most raccoon-proof post won't help.

In Wisconsin, the bluebird's nesting season mainly runs March through July. The birds generally raise two broods per season. Most monitors try to put up new nesting boxes in fall, or as soon as the ground thaws in spring. Open areas with short grass and widely scattered trees, such as lawns, country roadsides, mowed cemeteries and golf courses provide excellent bluebird box sites. The sites should be well away from woods, large trees and livestock feeding areas.

Suburban and rural property owners, especially government and corporate property owners, tend to be very happy to give permission for the erection and weekly monitoring of bluebird nest boxes.

Renewing an old nest box

It's a simple matter to convert paired deep boxes to smaller-volume singles. First, place blocks of 2x4 lumber inside at the bottom of the deep box until the top block is about five inches or less from the hole. Second, convert each pair of boxes to singles, by moving one of the boxes to a site at least 100 yards away from the other. (Simply plugging or taking down one of the boxes gives far less satisfactory results.) Third, switch to raccoon-proof posts. Follow these instructions and you're bound to double your bluebirding fun.

Become a bluebirder

It's not necessary to have a large quantity of nest boxes to enjoy monitoring bluebirds. A single Peterson nest box placed in my front lawn in Lafayette County has been producing double nestings of bluebirds for the last four years.

I get great pleasure out of sharing the discovery and surprises of nest box monitoring with children and grandchildren. When I have my grandchildren with me, I bring a short stepladder so the kids can climb up and see inside the boxes for themselves. They love it! Bluebird monitoring is a great project for schools, too – the investment required is minimal, and BRAW can provide volunteer coaches to guide the program.

Wisconsin has the land for many more bluebirds, and for many more bluebirders. A population of 10,000 bluebirds translates to an average of less than one bluebird every five square miles. The Bluebird Restoration Association of Wisconsin presently has about a thousand members, or about one member for every 54 square miles. We'd like to have more bluebirds and more BRAW members – and by using our new approach, we are confident that bluebird monitors will be twice as likely to witness bluebird-breeding success.

Joe O'Halloran chairs the data analysis group of the Bluebird Restoration Association of Wisconsin.