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Since we are going to have to suffer through the bad parts of the greenhouse effect, we might as well enjoy the good parts. Our sunny, warm, nearly snowless January may be a sign of our impending doom, but at least it will cut into the profits of Peoples Gas.

It is also providing us with more local sightings of Carolina wrens than we have enjoyed since the Ford administration. The Carolina wren is the largest of our eastern wrens, measuring a magnificent 5.5 inches from tip of beak to tip of tail, and it is also the most brightly colored.

Of course, "brightly colored" for a wren is not cardinal red or blue-jay blue. Wrens lean toward cryptic, mottled browns and grays, the sort of coloration that camouflages birds that spend their time flitting through the underbrush very near the ground. In this crowd, the buff underparts and rusty brown back of the Carolina wren appear positively gaudy.

The Carolina wren is one of six species of the family Troglodytidae in the eastern U.S. A troglodyte, as all Joan Crawford fans know, is a cave dweller, or more loosely, one who enters holes. The name may derive from the hole-nesting habits of the family's only Old World representative, the winter wren. Or it may refer to the hyperactive and highly inquisitive foraging behavior of wrens. A wren working a brush pile is in and out of every crack and cranny at a speed that makes the rest of the world look like it's doing t'ai chi. For birders, the biggest challenge in identifying a wren is getting a good look at one.

Fortunately, Carolina wrens sing a very distinctive song, a bright, whistled phrase usually described as sounding very much like "teakettle, teakettle." They sing year-round, so even winter birds can be identified by sound.

Carolina wrens nest in holes sometimes--including such unlikely holes as the pockets of old coats hanging in barns--but if no holes are available, they can weave a domed nest as a substitute. Their favorite nesting sites are around ground level in the same sort of dense brush they favor for feeding. A secure hole can be a year-round refuge for them, functioning as a nest in summer and a warm roosting place in winter. A warm roosting site is very important to the Carolina wren, because it is the only eastern wren that doesn't migrate. The others all move north in spring to nest and then fly back to the warmer south in fall.

Carolina wrens, for whatever reasons, have not developed this migratory habit. They do move around some in fall, but this seems to be the movement of young birds leaving the family home in search of a place of their own, not any general migratory movement.

There is considerable evidence that pairs of wrens that find suitable habitat may spend their entire lives in one small area, mating together each spring, and even staying together through the winter. At least one record exists of a banded pair that occupied the same piece of ground for five years.

Sedentary birds enjoy some advantages over migrants. They don't have to make long hazardous flights over territory devoid of food or shelter. They don't have to navigate through hurricanes or worry about running into the John Hancock building.

Their security carries a price, however. They can live only where they can survive year-round. They can't take advantage of the huge crop of mosquitoes in the north woods in the summer and then head for Louisiana when the leaves start to fall. And if things go bad on their home ground, they are more likely to starve or freeze where they are than to escape to some more hospitable place.

These limitations show plainly in the range of the Carolina wren. The northern end of that range extends from southern Iowa across northern Illinois, Indiana, and Ohio to Pennsylvania. Range maps show that limit as a precise line--a reflection of our culture's belief in exact boundaries--but for the birds, the northern limit is really a shifting zone that expands northward in warm winters and retreats south in cold winters. During warm spells earlier in this century, the species expanded temporarily into southern New England, only to be pushed back south when the weather turned colder.

Carolina wrens virtually disappeared from the Chicago area during the very cold winters of the late 70s. Our recent string of slightly warmer Januarys has allowed them to expand back north, providing us with sightings in Wilmette, among other places. If next winter turns cold, the wrens will probably vanish again. But if we really are living through the beginnings of the Greenhouse Era, Carolina wrens may be on their way to Lake Superior.

Temperature may be the controlling factor governing the fate of these animals, but snow is probably more important. Carolina wrens are insect-eating birds-- vegetable foods such as seeds and fruits make up only a very tiny fraction of their diet-and in winter, when insects and spiders are inactive, the wrens depend on pupae, hibernating adults, or overwintering eggs. Since they feed almost exclusively on or very near the ground, deep snows can simply cut them off from their food supply. Animals that are not getting anything to eat are much more vulnerable to cold, and there have been reports of frozen Carolina wrens. It takes fuel to survive zero weather.

A pair of Carolina wrens will typically nest two or even three times a summer, producing a brood of up to six young each time. The female incubates the eggs and does much of the feeding of nestlings. The male takes over the parenting when the young leave the nest, providing both protection and hunting instruction to the fledglings while the female begins sitting on another clutch of eggs. Their high reproductive potential has enabled them to reclaim their lost northern range almost as soon as the weather turned warm.

Another feature of the distribution of animals--and plants, for that matter--is that they are likely to be much more abundant in the heart of their range than they are out on the frontiers. The scarcity of animals at the edge of their ranges is partly the result of difficulties with the climate and partly the result of scarce habitat.

Both of these causes are at work in determining the territorial limits of the bobwhite, another species of bird that reaches its northern limit in this area. Bobwhites are ground dwellers and feeders like the Carolina wren, but they are much more catholic in their food preferences, savoring an enormous range of seeds, as well as insects and other small animals.

An old-fashioned farm, where the fields are surrounded by hedgerows and separated by woodlots, is ideal country for bobwhites. They nest in the shelter of the brush and move out into the fields to feed. Once upon a time, such habitat was common around Chicago, but there is little of it left today, and what we do have is often unavailable to bobwhites because they can't get to it.

Bobwhites are quail, members of the family of gallinaceous (chickenlike) birds that also includes pheasants, grouse, partridges, and turkeys. Gallinaceous birds are heavy bodied with short rounded wings. They are pedestrians, surprisingly fast runners but weak fliers who use flight almost exclusively as a means of escaping from danger. Flush a bobwhite or a pheasant, and it will explode into the air with a sudden burst of speed. But once aloft, it will quickly tire. A hundred yards is a long flight for such a bird.

Since they have to walk everywhere, most gallinaceous birds are extremely sedentary, but some of them do migrate, traveling on foot from high mountain meadows or exposed uplands to sheltered valleys. Bobwhites have been seen on such journeys. They will sometimes attempt to fly across major rivers only to fall exhausted into the water at midstream.

Lacking the grand mobility of more aerial species, especially in regions like ours where expressways and other very dangerous obstacles confine them to scattered islands, bobwhites don't bounce back quickly from hard winters. Most of the small amount of good bobwhite habitat we have is devoid of them, and barring deliberate introductions, their absence is likely to be permanent.

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