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Field & Street

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I suppose there is no really good time to sprain a toe, but for a birder the end of April is particularly bad. Here I sit, chained to my heating pad, my gait reduced to a painful hobble, while hosts of extraordinary rarities stream through Chicago on the spring migration. And the older I get, the more I am aware of the unalterable fact that I am only going to get a certain number of springtimes to enjoy this spectacle. I can't tape this show for later replay. If I miss it, I miss it forever.

Adding to my generally sour state of mind is the equally unalterable fact that the migration itself is in a state of decline. The neotropical migrants, the birds that winter in the tropics and nest in North America, are getting hit with heavy habitat loss and other problems at both ends of their migration routes.

Last week, when I was still possessed of ten healthy, unswollen toes, I did some birding along Rush Creek in northern Arkansas, and what I saw there set me to thinking more about the conservation questions that face birders--and everybody else. Rush Creek is a tributary of the Buffalo River. It is a perfect Ozark creek: clear and sparkling, dancing over a bed of clean gravel. Here and there, springs feed it. One I saw was flowing at such a furious rate that the whole surface of the stream seemed to be boiling. Elsewhere, seeps sent a slow trickle of water down the faces of rocky outcrops and into the creek.

There used to be a zinc mine along Rush Creek. About the time of World War I, 5,000 people lived in the town of Rush. The mine played out more than 50 years ago, but some of the old buildings are still standing--or rather, leaning--in the woods that have grown up since the diggings were abandoned. The forest is oak and long-leaf pine, with sycamores on the creek banks.

This land is now part of the Buffalo National River, a national park established after a long, acrimonious battle rather like the struggle that led to the creation of the Indiana Dunes National Lakeshore. Now a paved road leads from the state highway to the edge of the narrow, steep-sided valley cut by Rush Creek. Where the pavement gives out a gravel road continues on, dropping precipitously into the valley, crossing the creek on a low-water bridge and then paralleling it to its mouth. The road provides access to the Buffalo for canoeists and anglers.

The park service has installed a network of trails in the area, but the people who lay out hiking trails tend to assume that walkers are all after vistas, so they run their paths up to the ridge tops as quickly as possible. But ridge tops are the least interesting places for birders. We like to stay down in the valley, because water is a powerful lure for all sorts of wildlife.

So I birded along the half a mile of gravel road that parallels the creek, and I saw some excellent species. There were a few cerulean warblers in the treetops, and singing male parula warblers seemed to be spaced about 50 yards apart along the whole length of the road. I found red-eyed and white-eyed vireos, lots of blue-winged warblers singing their lazy, buzzy two-note song, a Kentucky warbler lurking in the underbrush, and a Louisiana waterthrush perched on a limb well off the ground so I could see its clear, white throat, a characteristic that separates it from the very similar northern waterthrush. Indigo buntings argued over territorial boundaries in the trees. From the woods on the other side of the creek came the ethereal song of the wood thrush, and in the crown of an oak just coming into flower I spotted a scarlet tanager glowing as if lit from within. Once, a Cooper's hawk crossed the valley, and soaring broad-winged hawks were often overhead.

What I was seeing along Rush Creek was a healthy community of eastern forest birds, something that is becoming ever more rare. I found forest-interior species like the wood thrush and the cerulean, and along the edges of the opening created by the road, the parulas and blue-winged warblers. I also saw a couple of cowbirds. The brown-headed cowbird is a brood parasite, a species that lays its eggs in the nests of other birds and leaves the young to be raised by the foster parents. Cowbirds live in open country or along woodland edges, so wood thrushes, cerulean warblers, and other birds of the forest interior did not encounter them in the days when vast tracts of unbroken forests covered much of the eastern U.S. But apparently, even a dinky little two-lane gravel road is enough of an opening to let them into the forest along the Buffalo. It is sad to see them there, because the forests of the Ozarks are generally thought to be big enough and unbroken enough to produce a population surplus of species such as the wood thrush. They are a refuge that helps keep this species alive in an otherwise hostile environment. It is interesting that both the blue-winged warbler and the parula, the two most common warblers along Rush Creek, are for whatever reason rarely parasitized by cowbirds. Perhaps as birds of edges and young forests they have evolved some defenses against a parasite that has shared their habitat for a long time.

The blue-winged warbler is a bird whose history over the past couple of centuries is an object lesson in the complexities of nature. The scientific name of this bird is Vermivora pinus. It is very closely related to the golden-winged warbler, which is Vermivora chrysoptera. In fact, the relationship is so close that where the ranges of the birds overlap they freely hybridize, producing two distinct types of offspring. The genetically dominant form, called Brewster's warbler, looks like a blue-winged except that it has the white underparts of the golden-winged. The recessive type, called Lawrence's warbler, has the black cheek and throat patches of the golden-winged, but its underparts are yellow like the blue-winged.

Both of these birds live in forests in early successional stages, when young trees are just beginning to take over open land. Early American ornithologists like Alexander Wilson and John James Audubon hardly ever saw either species. In the days when virgin forests dominated eastern North America, both of these birds were very rare. But the cutting of America's forests opened up enormous opportunities for both species; forests cut over and then allowed to regrow were ideal habitat. As farmers moved westward into the rich lands of the midwest, farms on the rocky soils of New England began to be abandoned. And as forests returned to these old fields, blue-winged and golden-winged warblers grew ever more abundant.

Then a strange thing began to happen. All across eastern North America, the blue-winged warbler began to expand its range at the expense of the golden-winged. Originally, the blue-winged lived mostly west of the Appalachians. It is likely that the species originated in the Ozarks, and its center of abundance was in the valleys of the Mississippi and Ohio rivers. The golden-winged was a northerly species living in northern Illinois, Wisconsin, Michigan, and the higher elevations of the Appalachians as far south as Georgia. In Illinois the border between the two species was 100 to 200 miles south of Chicago.

Frank Gill of the Philadelphia Academy of Natural Sciences has been studying the interaction between the two species. According to him, blue-winged warblers typically advance northward into a region where golden-winged warblers live. Hybridization becomes common along the frontier, and then, in as little as 20 years, the golden-wings disappear.

In the most recent edition of Peterson's Field Guide to the Birds, published in 1980, the range maps for the two species showed the frontier between them running right through the Chicago area. But now the blue-winged warblers have nearly reached the northern border of Wisconsin and the golden-winged have totally vanished from this region. Some of their genes still survive in populations of blue-wings, however. They show up in the occasional appearance of Lawrence's warblers. Since the genes that make a Lawrence's are recessive, blue-winged warblers can carry them without showing them. If two carriers get together, they can produce the homozygous recessive genotype that makes a Lawrence's warbler.

Dr. Gill's interpretation of this story is that geographic isolation of two populations of the ancestor of V. pinus and V. chrysoptera started the process that creates new species. Things had advanced far enough to give the two forms distinctive plumages, but not far enough to prevent interbreeding. When huge habitat changes threw the birds back together again, they recombined, with the blue-winged physical type displacing the golden-winged type.

The golden-winged may survive in the north if something in the blue-winged form makes it impossible for it to live in the cooler climate. In the Appalachians golden-winged warblers remain on the ridge tops while the blue-winged are confined to the valleys and lower slopes. And in northern New Jersey and the Catskills, Dr. Gill says, "something may be happening." In those places the two species coexist and do not hybridize. The situation seems stable, with no replacement of golden-winged by blue-winged. It may be that the populations in that region have actually become separate enough to be considered true species.

For us, the thing to keep in mind is that this whole process, a process that may take many more decades to play itself out, is happening because more than a century ago we cut down a lot of trees.

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