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Almost nobody has anything bad to say about bluebirds. They are birds of happiness--gentle of demeanor, sweet of voice, confiding of disposition, beautiful of plumage, colored, it is said, like the sky above and the fresh turned earth below.

They also have a very high tolerance for human beings. It seems likely that they nested around Indian towns long before Europeans began coming here. When the Pilgrims landed at Plymouth Rock in 1620, bluebirds practically met them at the shore, and they quickly became a common nesting bird around the colony.

The immigrants named them blue robins because of their resemblance to the robin' redbreast of England. Both birds are members of the thrush family, as is the American robin, and both are small and plump bodied with a shape that gives them a hunch-shouldered look when they perch. Both have brownish-orange breasts and white bellies. But the English robin is brown backed and the bluebird is that marvelous blue, a deep, rich blue that the sky rarely attains.

Until well into this century, bluebirds were among our most common songbirds. They nested around farms, in small towns, and in the quieter neighborhoods in cities. Their arrival in the north in early March was a harbinger of spring and one of the few bits of color in the drab winter landscape. But a lethal combination of circumstances has, in a few decades, reduced the population by as much as 90 percent and made the bluebird one of our rarer species. Bluebirds are now, it seems, making a modest comeback, with some help from human beings, the creatures who got them into trouble in the first place.

The eastern bluebird, to give it its full name, ranges over North America from the Atlantic to the foothills of the Rockies. It nests as far north as James Bay in Canada, and it is a resident bird in Mexico and Central America as far south as Honduras. It has two close relatives, the mountain and western bluebirds, that occupy the continent from the Rockies westward.

Bluebirds are creatures of the woodland edge. They need the trees as nest sites and song perches, but they hunt on open ground. In primeval Illinois, they doubtless lived in the savannas, the grasslands dotted with scattered trees that were a common feature of our presettlement landscape.

The arrival of Europeans in North America was a real boon to bluebirds, as it was to many edge species. Settlers cut into the dense unbroken forests of the east, opening them up, creating miles and miles of new edges bordering plowed fields and pastures where the bluebirds could hunt for grasshoppers and beetles.

The settlers also brought cows and horses and sheep and pigs, animals that needed fences to keep them out of the cornfields. Fences meant fence posts, and wooden fence posts--especially old ones with a bit of rot hollowing out a cavity near the top--were ideal nest sites for eastern bluebirds.

Bluebirds are the only members of the thrush family to build their nests in cavities rather than out in the open. In nature they depend, as do many other birds, mainly on woodpeckers to do their woodcutting for them.

Birds that nest in cavities generally enjoy a lower infant mortality rate than open-nesting species. The young are safer from both bad weather and predators. Bluebird babies are completely naked when they hatch. They don't have to invest any energy in the coat of down that baby robins or song sparrows need to save them from cold.

The bad side of cavity nesting is that there are rarely enough cavities to go around. Competition for nest sites can get quite intense, and in most years a certain percentage of the adults won't be able to breed even if food is abundant.

There is also a lot of competition between species for the available cavities. Tree swallows and house wrens are the bluebirds' most common native competitors. The similar needs of the three species probably cut into the populations of all of them, but the birds have evolved ways of minimizing the harmful effects of direct competition. Wrens tend to prefer brushier places than bluebirds, and that habitat preference cuts down the occasions of conflict.

Migrant tree swallows usually begin arriving here at the end of March, and the later arrival puts them on a different nesting schedule than the bluebirds.

However, there are no such separating mechanisms operating with house sparrows and starlings. Competition from these two foreign invaders is a major element in the triple whammy that has nearly done in the bluebird.

Both of these species are hole nesters, and both of them compete directly with bluebirds for nest sites. Bluebirds are larger and stronger than house sparrows, and one-on-one, a male bluebird could probably drive off the smaller bird. But house sparrows are rarely alone. Ornithologists use the term "mob violence" to describe the methods that flocks of house sparrows use to drive away any bird that challenges them for a choice nesting site.

Starlings are big and strong enough to drive out bluebirds without calling in a mob to help, and their arrival on the scene has done much damage to the native species.

The second element in the triple whammy is that old familiar favorite, habitat loss. Managing a woods for maximum production means cutting down dead or sickly trees and thus eliminating the places where cavities are likely to develop. Farmers don't let trees and shrubs grow along fence lines anymore. Removing the shade cast by a hedgerow allows them to plant an extra row or two of corn.

Even the fence posts are gone, replaced by steel posts or removed completely because many contemporary farmers don't keep animals.

The final leg in this disastrous triad is a reduction in the food supply. Modern farmers spray their fields with pesticides, so there are far fewer insects to eat than there used to be.

Besieged on three fronts, the eastern bluebird had no place to go but downhill. Just 30 years ago, it was a common nesting species in the Chicago area; now it is a rarity. We had only one known nesting pair in Cook County last year, on a farm near Barrington, and this year, birds on the same site lost their first clutch of eggs and departed.

If you would like to see some bluebirds, Ryerson Woods is a good place. At this Lake County Forest Preserve on Riverwoods Road in Riverwoods, the staff has put out homemade nesting boxes, and some bluebirds have shown up to nest in them.

Nest boxes put out by bluebird lovers are playing a major role in the modest comeback that the species seems to be making. Several designs are in use. The most popular is called the Peterson Bluebird House. The name comes not from Roger Tory Peterson, the bird artist and field-guide author, but from a man in Minnesota who invented the design.

A Peterson nest box is a four-sided wooden structure with an entrance hole located high on one side. The front and back are rectangular. The sides taper, making the nest cavity wider at the top than at the bottom.

The roof slants steeply, with the high end at the back and the low end over the entrance hole. The eaves extend well out beyond the walls. The entrance hole is either a circle an inch and a half in diameter or an oval measuring 1 3/8 by 2 1/4 inches.

All of these features are products of much thought and experience. The steeply slanting roof with its broad eaves makes it hard for cats and raccoons to sit on the roof and reach in the entrance hole. The size and shape of the hole itself prevents starlings from entering. The tapering nest cavity is a feature the bluebirds like. Wood used in construction should be one-inch boards to provide insulation. There should be no perch by the entrance hole. Bluebirds don't care for them. House sparrows love them.

You mount the house on a post four to six feet from the ground. Mount a flat tin plate on the post, or grease it, to keep out squirrels and snakes, and you are in business.

Most bluebird lovers build many houses and mount them at intervals along country roads or near footpaths. Bluebird trails, as they are called, are operating all over Illinois, providing refuges where the birds can safely reproduce. Birds fledged in the nest boxes provide the numbers we need to recolonize the small amounts of natural habitat that are available.

If you are interested in building houses for bluebirds, you can get complete plans and instructions from the Illinois Department of Conservation, Forest Resources and Natural Heritage Division, 524 S. Second St., Springfield, IL 62706.

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