When John Terborgh was a child his family lived in the country near Washington, D.C. Their house stood on two acres of abandoned farmland fronting a dirt road that dead-ended two doors down. Behind the house, a path meandered through almost a mile of woodland, crossing a quiet stream along the way.
The woods shaped his life. With few children his own age nearby, he spent his days in the woods, fishing in the stream, searching for snakes and salamanders, and, especially, watching birds.
He was in the right neighborhood for that. He could always find red-shouldered hawks across the lane, and the woods provided a nesting ground for yellow-billed cuckoos, scarlet tanagers, red-eyed vireos, wood pewees, hooded warblers, acadian flycatchers, and whippoorwills. As a teenager in the 50s, he put together a list of 150 species from the land around his house; one species the list did not include was the house sparrow, that boring indicator of human habitation.
Today, the area around Terborgh's childhood home has changed radically. It is now a part of suburban Washington. The dirt lane is a major street, and a housing development occupies the land where the red-shouldered hawks lived. But the path through the woods is still there. The woods is a county park, and in some respects it looks the same as it did 40 years ago. The path now crosses three roads along its length, but otherwise the only obvious change is that the trees are bigger.
Despite its protected status, nature does not thrive in the woodland. Once, ten different species of snakes could be found there; now almost none remain. Box turtles used to be so abundant they were serious pests in the Terborgh family garden. Now a sighting of a box turtle is a rare occurrence.
The bird population has been similarly reduced. You can still find mockingbirds, robins, woodpeckers, and chickadees, but the vireos, warblers, cuckoos, and tanagers have disappeared. Why? Terborgh addresses that question in a new book called, appropriately enough, Where Have All the Birds Gone? (Princeton University Press, $45 hardbound, $14.95 paper).
Terborgh is an ornithologist on the faculty at Princeton and a leading figure in the recent flowering of knowledge about the bird life of the American tropics. His book is mainly about the difficult task that scientists face in trying to answer that title question.
To begin with, we can't really say whether the missing birds ought to be in that woodland. The almost total destruction of the natural landscape east of the Appalachians has left us without a control group, without a past to measure the present against.
Second, how can we be sure that we are not seeing a strictly local phenomenon? Maybe the birds have left this one particular woodland but are doing fine elsewhere. Maybe their absence means nothing.
Monitoring populations of wild animals is never easy, but some are more difficult to keep track of than others. Whooping cranes are big birds of open country and the entire breeding population consists of a single flock. We can know the whooping crane population to the bird. But scarlet tanagers are small woodland birds whose breeding territory extends from Nova Scotia to Oklahoma and from Manitoba to South Carolina and whose winter range extends from Panama to Peru. How can we keep track of them?
Terborgh discusses some of the attempts we have made, including the surveys run by the U.S. Fish and Wildlife Service and the breeding-bird censuses sponsored by the National Audubon Society. Both these attempts have their weaknesses, but they do tell us enough to let us know that the decline in many bird populations is a widespread and general phenomenon. The species being hit the hardest are long-distance migrants like the vireos, warblers, thrushes, and tanagers.
I have written about this problem before, most recently in an account of Scott Robinson's surveys of cowbird parasitism among Illinois songbirds. Cowbirds are brood parasites that lay their eggs in the nests of other species, leaving their young to be raised by the foster parents.
In primeval North America, cowbirds were mainly birds of the open plains, but the fragmentation of the eastern woodlands allowed them to expand their range. They have also been helped by mechanized grain harvesting, a practice that leaves enough corn, wheat, or soybeans in the fields to supply cowbirds with winter food.
These changes have given cowbirds a chance to parasitize birds of woodland interiors, species with none of the defenses against parasitism evolved by birds of the woodland edges. The long-distance migrants are particularly vulnerable because they only have time to nest once or twice a summer. Sedentary birds and short-distance migrants may nest four times a year, and thus have twice the opportunity to reproduce successfully.
Small suburban and rural woodlots also give predators a much greater chance of finding a nest. Terborgh reports on an ingenious experiment conducted by a Princeton graduate student named David Wilcove to test predation rates. Wilcove made artificial nests and filled them with domesticated quail eggs. He then placed the nests in woodlands, some on the ground, some in low branches.
Wilcove may not have been as clever as a bird in hiding his nest, but his results were shocking. In suburban and rural woodlands, rates of destruction were near 100 percent. The destroyers were raccoons, opossums, feral house cats, crows, and blue jays, among others.
By contrast, only 2 out of 100 nests placed in the heart of the east's largest remaining old-growth forest, in Great Smoky Mountains National Park, were attacked by predators. Terborgh believes that our few large woodlands are serving as reservoirs for our long-distance migrants. Excess birds from places like the Smokies move out to colonize smaller woodlands where they are unable to reproduce in sufficient numbers to sustain a population. In other words, our situation is even worse than it appears, because the birds we see in smaller woodlands are probably not part of a self-sustaining population. They are wanderers who have just come there to die.
Our long-distance migrants are also getting hit on their wintering grounds by the enormous deforestation going on in the tropics. Terborgh has been a major figure in the growth of Neotropical ornithology in the past two decades. He tells how the publication of good field guides combined with the jet airplane to vastly expand the opportunities for fieldwork in Mexico, the West Indies, and Central and South America.
Because of all this fieldwork, we have a much clearer idea of how to protect the 250 North American species that spend their winters in the tropics. Unfortunately, our greater knowledge has mainly let us know how complicated everything is.
For example, some species establish individual territories for the winter. Both males and females create and defend these territories; in some cases, the females even sing the species's song to declare their sovereignty over a particular patch of ground. Other species winter as pairs, often joining in mixed flocks with resident species. Hooded warblers segregate by sex, with the males occupying old-growth forests and the females concentrating in young second-growth.
Having surveyed our increasingly desperate situation, Terborgh makes a number of recommendations for things we can do here and things we can help Latin Americans do there to keep our birds alive. Here, we need to remember that in designing preserves of various kinds, bigger is always better. We need to return to the multiple-use management policies and sustained-yield logging practices that were the founding principles of the U.S. Forest Service, which means turning away from the subsidized timber sales and intensive tree farming that Reaganites have been pushing on us for the past decade.
In Latin America, we need to direct our aid policies toward urban development rather than colonization of new lands. According to Terborgh, the average settler in the tropical forests is someone who left the city because he could not make a living there. Few people would choose the very hard life of an Amazonian subsistence farmer if they had any real choice in the matter.
And we should encourage the application of our National Forest idea to the tropics. According to Terborgh, nearly all the land suitable for agriculture is already being farmed. Clearing forests to plant crops or graze cattle is futile and destructive. An ecologically based, sustained-yield silviculture is the best use we can make of the remaining tropical forests.
Where Have All the Birds Gone?, like Silent Spring and A Sand County Almanac, is a book that will profoundly affect the way we think about nature and how we can best preserve it.