In early September the woods are quiet. The birds, through nesting, no longer sing, but the leaves are still on the trees, damping the sounds of traffic from the roads. There's an atmosphere of waiting.
That's how it was at Dennis DeCourcey's bird-banding project one recent Saturday: long stretches of waiting punctuated by brief periods of activity. DeCourcey expected it to be a good day because a cool north wind had ended a hot spell late the day before--the sort of wind that can bring in waves of migrants. After such a wind stops blowing the songbirds that have ridden it south will settle into woodlands to rest and feed before continuing their journey.
Here they would settle into Waterfall Glen Forest Preserve, a doughnut-shaped woodland that surrounds Argonne National Laboratory near southwest-suburban Lemont. DeCourcey, a free-lance ornithologist, has been banding birds in the preserve for the last five years, trying to find little bits of answers to very large questions. For example: To what extent do migrating songbirds rely on the woodlands where they stop over, especially for food? How long do they stay there? How do high suburban deer populations affect the populations of nesting birds?
DeCourcey, a stout, jovial 50-year-old, traps the birds he bands in a densely wooded part of the preserve in about ten "mist nets." The black plastic of which they're woven is fine and hard to see, and I nearly walked into several of them, so neatly do they blend into the surrounding foliage. It's a good thing I stopped in time--they cost $95 apiece. Each net is roughly 30 feet long and eight high and has horizontal folds so that birds that fly in will, in effect, fall into a mesh bag.
DeCourcey operates the banding station one or two days a week, May through October. In a typical year he bands up to 1,000 birds. On this Saturday he was joined by about ten volunteers--far more than necessary, but then he's an enthusiastic raconteur and teacher who likes to share what he knows.
Birds are most active in the morning, so we meet at 7 AM. DeCourcey and the others walk around the plot and unroll the nets, which are furled into tight black ropes between aluminum poles. He points out that most of the leaves on the shrubs and trees around the nets start about six feet above ground--the browse line. Deer have eaten everything below that.
Because there are so few plants on or just above the forest floor, there's less cover for birds. "We expect to see a positive correlation between the understory and ground-cover density and the bird population," DeCourcey says. So far that seems to be the case. When he began monitoring five years ago the local deer population was estimated at 32 per square mile. By 1993 it had shot up to 142 per square mile, and in the summer ovenbirds and wood thrushes--both of which nest on the forest floor--had vanished. DeCourcey also found several birds that had been killed while trapped in the nets, and he speculated that deer might have begun to eat them. Last winter the deer numbers were reduced to 32 per square mile again through a culling program, and DeCourcey is waiting to see whether the birds return as the vegetation grows thicker.
By eight o'clock all the nets have been up for about half an hour. I walk the well-worn trail that connects the nets with Leslie DeCourcey, Dennis's wife and an experienced bander, and Deb Maklezow, a volunteer who learned of the project while taking an ornithology course at Northeastern Illinois University. Only one net has a bird in it. It's a medium-size bird with a dark gray back, a white belly, and a large ring around its eye--a Swainson's thrush.
Maklezow is learning the banding business, and Leslie DeCourcey talks her through the process of removing the bird from the plastic webbing in which it's entangled. "First get out the one wing that's not too caught. Next get the head out. OK, now the other wing." Maklezow slowly disentangles the thrush and holds it, grabbing the thin legs between her fingers and letting the head project between her thumb and middle finger. The bird shits a dark blue liquid all over her palm. "It's been eating berries!" she says, then puts it in a small cloth sack.
Back at camp, actually a picnic table set up in the woods, the bird is taken out of the bag by Glenn Gabanski, a high school teacher who's also a board member of the Chicagoland Bird Observatory, the nonprofit organization DeCourcey set up to support this and other bird-monitoring projects.
The thrush struggles, squawks, and pecks at Gabanski's hand. He uses a special pair of pliers to put the narrow aluminum band around the bird's right leg, then examines the bird's breast to see whether it has built up any fat reserves, which can be seen through the thin skin. These fat deposits fuel a bird's long migratory flight. A bird with little or no fat has probably arrived recently at the woodland and may stay a while to feed; a bird with good fat deposits has perhaps been feeding there for a few days already. On a day like this, after a north wind, DeCourcey expects mainly birds with little fat. With the wind at their backs they may have flown a long way.
Gabanski also scrutinizes the bird's wing feathers for subtle buffy markings that indicate whether the thrush is an adult or was born this year. He wets the feathers on the bird's head and examines its skull. Because the bones that make up the skull only gradually grow together during a bird's first year, this is a means of double-checking its age. Often it's a tough call, and DeCourcey, who has the most experience, decides what to record.
In the fall, when many songbirds have shed their gaudy breeding plumage for more camouflaged tones of green and gray, even identifying the species of a bird can be difficult. More than 20 species of warblers can be found in the forest preserve, and many look dismayingly similar--small, drab, olive colored. Later in the day Gabanski and Maklezow will agonize over a bird for about ten minutes before Leslie DeCourcey identifies it for them as a Cape May warbler. She's an artist whose trained eye serves her well in the field.
The data is all recorded in a notebook and later will be transferred to a computer and sent to the National Biological Survey in Laurel, Maryland. If a birder later finds a dead banded thrush that has, say, flown into a window in Mississippi, all he has to do is read the tiny number on the leg band and report it. The Fish and Wildlife Service will connect the dots, and another tiny bit of information will be added to what's known about migratory routes and the survival rates of birds.
It's not a new idea. The first bird bander in the Americas is generally thought to have been the great ornithologist and artist John James Audubon, who tied strands of wire around the legs of several phoebes at his Pennsylvania farm and was gratified when the same birds returned the following spring. He already knew that birds migrate, but this was his first confirmation that they might return to the same nesting territory year after year.
Since then banding has become a large-scale enterprise. Thousands of banders are active in the United States. Some are academics, some are government biologists, some are interested lay people. At most recent count, 112 people in Illinois held banding permits from the Fish and Wildlife Service. A total of more than 50 million birds have been banded in North America.
Only a small percentage are ever recovered. The recovery rate for many migratory songbirds is especially low--averaging about 2 percent--since they're small and often die in remote forested areas. Still, over the years banding has revealed a great deal about their migratory patterns.
Compared to birds that reside in one area year-round, migrants are subject to many pressures. They must find not only safe places to nest, but also safe places to rest and feed during migration and safe wintering grounds. Many of the species that winter in Central and South America, known to ornithologists as neotropical migrants, have been declining. In Illinois ovenbird populations have dropped 70 percent since the mid-1960s, yellow-billed cuckoos 56 percent, and red-eyed vireos 36 percent (these figures are based on the Fish and Wildlife Service's annual springtime Breeding Bird Survey). These drops have occurred even as the total acreage of forest land in the state has increased.
Because the area of forest lands in the United States has risen or stayed roughly constant, many biologists have blamed such population declines on the clearing of the virgin forests that constitute the primary Latin American wintering grounds. But DeCourcey's work, along with that of hundreds of other researchers, seems to show that this explanation is too pat. Changes in forest structure in North America may be as significant as deforestation in the south, perhaps even more significant. Deer overpopulation and the resulting changes in vegetation are only one example. Forests in the U.S. have also become more fragmented--extensive woodlands have been logged or turned into archipelagos of small woodlots.
Many birds that thrive in the deep woods, among them wood thrushes, can't reproduce well in small wooded areas. Small predators like raccoons and crows are more abundant along forest edges, and many woodlands in Illinois are so small that they are in effect all edge.
Brown-headed cowbirds also thrive in agricultural areas and along edges. These nest parasites, as biologists call them, build no nests of their own but instead lay their eggs in the nests of other birds. The young hatch and, being larger, faster-growing, and more aggressive than the young of the host species, are usually the only ones raised by their foster parents. Research conducted in southern Illinois' Shawnee National Forest by Scott Robinson of the Illinois Natural History Survey has revealed that over 90 percent of the wood thrushes breeding in some areas are raising baby cowbirds. Wood thrushes typically live only a few years, so if they reproduce this unsuccessfully they may be wiped out of an area in a short time.
Given so many variables, it can be extremely difficult to figure out just why a given population or species is declining. A program like DeCourcey's can help. Say wood thrushes return to Waterfall Glen and begin nesting again. If they reproduce successfully, then young wood thrushes should blunder into the nets periodically. And if they do while the overall population still declines, then the habitat problems probably lie elsewhere. If no young thrushes appear, then some factor at Waterfall Glen must be inhibiting reproduction.
But that would still be only a snapshot. To see the big picture you need information from many sites, which is why the Waterfall Glen project is only one of about 250 stations in the U.S. and Canada participating in the California-based Institute for Bird Populations Monitoring Avian Productivity and Survivability project. The researchers at these stations send their annual banding data to the institute, which puts it together and analyzes population trends on a regional basis. Over the years this network may accurately identify not only trends, but causes.
Habitat destruction and modification are problems of such scope that it is, DeCourcey says, "hard to be optimistic. There are just too many things that can't be measured. There aren't enough biologists in all the world to answer all the questions that are arising. That doesn't mean that any of these species are going to become extinct. But I think it may mean that we'll have to travel 150 miles or more to see warblers that we can see now, in spring, in Jackson Park."
On a warm Saturday morning in September these concerns seem distant. We're out enjoying a pleasant morning in the woods, watching Gabanski finish working on the Swainson's thrush. Later in the day another 20 to 25 birds will be caught, including eight or nine species of warblers--a fair haul. But as we look at the little feathered bundle Gabanski's holding, there's only astonishment that a body so small can make it to South America for the winter. It's a little hard to remember that the birds have been managing these long journeys for millennia without any help from us. As the bird wings away from Gabanski's opened hand and flies swiftly into the shelter of the woods, I'm reminded again that it's healthy to be guided by a certain amount of faith in what the natural world does when we're not looking.
Art accompanying story in printed newspaper (not available in this archive): photos/Cynthia Howe.