In the Shawnee National Forest the red-eyed vireos are getting scarcer, the cerulean warblers are nearly extirpated, and the remaining wood thrushes are raising cowbirds.
In deepest southern Illinois, where the Shawnee holds our state’s largest tracts of forests, forest-interior birds like the wood thrush can be found raising broods that are nothing but cowbirds, and the average wood-thrush nest contains almost twice as many cowbird eggs as wood-thrush eggs. Ever larger areas are devoid of red-eyed vireos, and only a few ceruleans remain, despite an abundance of suitable habitat for the species.
We know these distressing facts because of a massive, multiyear study of the nesting success in the Shawnee of neotropical migrant forest-interior birds being conducted by Dr. Scott Robinson of the Illinois Natural History Survey. Robinson has deployed a small army of student workers to do the fieldwork for his study. Toiling for burger-flipper wages in a climate that tends to make you feel like you are standing directly under the exhaust vent of a clothes dryer, they rise at dawn to spend long days finding nests, monitoring nests, and banding birds. After three years in the field, Robinson’s army has accumulated a data set that tops anything else available by a factor of three or four.
And what the data reveal is a picture of the lack of reproductive success of many species of neotropical migrant forest-interior birds. These birds are hurting in part because of the loss of winter habitat caused by the destruction of tropical forests, but their problems on their summer range seem to stem from the fragmentation of the local landscape, which exposes these birds to the hazards of life on the forest edge, particularly to high rates of predation–by raccoons, opossums, and other animals–and brood parasitism by the brown-headed cowbird. Nothing in the evolutionary histories of the forest-interior birds has prepared them to face such hazards. They lack the defensive strategies of such edge species as the gray catbird–which removes cowbird eggs from its nest–and their long migrations shorten their breeding season so that they cannot adopt the strategy of song sparrows and cardinals, which may nest as many as five times each summer.
The Shawnee project grew out of a study Robinson began in 1985 at Lake Shelbyville, a large artificial lake south of Decatur in central Illinois. Robinson describes the landscape around the lake as “hacked up beyond belief.” It is cornfields and houses and scattered woodlots, none of them of any size. Wood thrushes were nesting in the woodlots, but when he surveyed their nests he found parasitism rates almost beyond belief: as many as 11 cowbird eggs in one nest, and many nests with nothing but cowbird eggs.
Robinson concluded that he had discovered a population sink, a place where wood thrushes came to nest, to raise cowbirds, and to die without passing along their own genes. But if Lake Shelbyville was a population sink, where were its forest birds coming from? The Shawnee National Forest, the nearest large wooded area, seemed the most likely place to look.
The Shawnee is small as national forests go. The land within its boundaries totals nearly 840,000 acres, but only about 265,000 acres of that are actually owned by the forest service. The rest is controlled by other public agencies or by private owners, and it includes cornfields, factories, and even whole towns as well as forest lands.
Still, if there is anywhere in the state where birds of the forest interior could escape edges, this should be it. Robinson hoped that a study of forest-interior birds in the Shawnee would reveal what the various species need to thrive. “I thought forest fragmentation was going to be a simple matter of distance to edges, extent of the edge, and size of the tract,” he says. “I hoped to develop a series of local-management recommendations on how to improve bird productivity in the forest by minimizing edges. I wanted to fulfill my obligations to the taxpayers by coming up with something that would actually benefit Illinois.”
But as the data began to accumulate–as Robinson and his army of minimum-wage nest finders combed the woods seeking the nests of wood thrushes, Acadian flycatchers, summer and scarlet tanagers, and Kentucky, hooded, worm-eating, and cerulean warblers–a picture began to emerge of pervasive parasitism and high predation, of a landscape that was all edges.
Not all species were equally affected by the situation. While wood thrushes seemed to be heading for a catastrophe, worm-eating warblers–a species that studies in the east had shown to be extremely sensitive to landscape fragmentation–were doing rather well. So were Kentucky warblers, although neither they nor the worm-eating warblers were as successful as other studies had shown them to be in large tracts. The study had begun as a way of measuring edge effects and finding ways to minimize them, but the surprises in the data were raising complicated questions about what species were doing well and what kinds of places they did well in.
The topography of southern Illinois consists of clumps of low hills characterized by narrow ridges and steep-sided ravines. The hills hold the forests where Robinson’s crews search for nests. Between the ranges of hills the land is either rolling or very flat. The Mississippi is just a few miles west of Robinson’s research site, and the Ohio joins it just one county south. There are oxbow lakes on some of these flat lowlands, along with a few cypress swamps that have escaped the plow and the ax. But most of the land is given over to farming.
“We had a landscape that was naturally fragmented because most of the forests were riparian,” Robinson says. “It was fragmented by fire, so you had luxuriant, dense forests where it was wet and on the dry ridge tops there was probably a lot of fire and a kind of open forest.”
Agriculture has been the biggest source of fragmentation created by humans, but even within the forests the land has been deliberately carved up. “Until recently, forestry practices involved clear-cutting, which creates lots of 15- to 25-acre gaps in the forest. And traditional wildlife managers, which is to say game managers, created wildlife openings so there wouldn’t be large blocks of unbroken habitat; breaking up the habitat was good for the basic game animals–deer and turkey.
“All these things were happening simultaneously in a landscape where there really aren’t any huge tracts. You can’t say how big a tract is. You look at a map, you see it is really a whole series of interconnected patches with openings in between. It’s an interconnected, complex landscape in which no area is large enough to escape the negative effects of edges: high predation and high brood parasitism.”
When he began to study the Shawnee, Robinson expected to find that agricultural openings would be worse than clear-cuttings. He thought cowbirds would stick close to pastures and raccoons and opossums would be near cornfields, but he discovered that he was wrong. “We are absolutely full with all those animals, especially the cowbird. They are just at saturation level.”
I spent a day in early July sifting through the piles of nest records looking for some sort of overview of what was happening in the Shawnee. For the wood thrushes the picture was indeed bleak. The 89 nests that had been found contained a total of 115 wood-thrush eggs and 198 cowbird eggs. Just 1.3 wood-thrush eggs per nest, and this before the predators got to them. And the predators were very active. Reports by nest monitors were filled with depressing accounts of “no adults,” “broken eggshells on ground,” “nest empty.” Or this poignant note on a Kentucky warbler nest: “female dead three feet from nest; some feathers scattered, fractured beak in nasal area, hole in breast. Eggs intact.” The pile of nest logs left me with the feeling that the lives of wild birds are so hazardous it’s a miracle any of them survive.
However, there were still 89 wood-thrush nests discovered in the study area, so the birds have not completely vanished. But Robinson notes that there are not enough birds to fill the available habitat. “Species like the wood thrush and the red-eyed vireo tend to gather in clusters,” he says, “leaving large areas of suitable habitat with none of these birds. On the other hand, the yellow-throated vireos seem to spread out through the forest, so they are uncommon but present in all suitable habitat.”
A habit of gathering together, of breeding in clusters, could be helpful to a bird whose population is in decline. Individuals might have a better chance of finding mates than they would if they were spread throughout the forest.
The study is also beginning to clarify the effects of both predation and brood parasitism. Heavy predation reduces populations by driving birds away. They will not return to an area if their nests are repeatedly destroyed there. Birds will return to nest in a place where they raised nothing but cowbirds the previous year because they think they had a successful breeding season. Obviously, if heavy parasitism continues, the population will eventually crash unless new birds are coming in from somewhere else. Robinson suspects that one of his clusters of wood thrushes–in a place called Ripple Hollow–is about to suffer such a crash. Dutch Creek, another study area, has already crashed, dropping from 20 nests the first year of the study to 2 last year.
Further details will have to await a more extended analysis of the data and what Robinson hopes will be two more years of study. “A five-year study will level out some of the year-to-year variations and give us a clearer picture,” he says. His fear is that the picture will be a glimpse of the future everywhere in eastern North America as habitat continues to be fragmented.
At this point, Robinson says, “I feel confident in making two recommendations. First, in the Shawnee at present, there are no tracts large enough to provide protection from edge effects, but there are some places that would be large enough if land were purchased strategically and if the clear-cuts and wildlife openings were stopped. Second, the larger national forests, those which have big tracts, should recognize the global importance of those tracts and preserve them.”