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The Case of the Missing Songbirds
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Updated 11 November 2004

Volume 3, Number 1, 1997
 

 

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The Case Of The Missing Songbirds


By Scott K. Robinson

The familiar songbirds that build their nests in our yards and trees and meadows are the most watched, and probably the most beloved of all wild creatures. They are also among the most traveled, and therefore shared, for most of them spend their allotted years shuttling back and forth between their summer and winter homes, often following long-established flight paths that take them over thousands of miles of land and water.

The migratory songbirds found in North America include roughly 350 species, of which about 250, known as Neotropical migrants, spend their winters in the New World tropics of southern Mexico, Central and South America, and the West Indies. The other 100 species, called short-distance migrants, winter chiefly in the southern U.S., particularly along the Gulf Coast. Migratory songbirds can be found in virtually every habitat on the continent, and usually half or more of the breeding birds in any sampled area are migratory. In some northern forests, for example, less than 10 percent of the songbirds present in the summer remain throughout the year: the rest winter in places far away.

Migratory songbirds play a major role in the health and functioning of ecosystems, as consumers of insects (especially those that defoliate trees), dispersers of seeds, and pollinators of flowers. They are also of considerable value to regional economies. When forest birds eat insects, the result is greater tree growth and a longer period between insect outbreaks -- services that may be worth as much as $5000 per year for each square mile of forest land. Millions of people watch birds as a hobby and many of them flock to areas where birds concentrate, where they spend millions of dollars on ecotourism.

In the spring of the year, mostly from March through May, migrants abandon their wintering grounds, fly northward in the dark of night over several thousand miles, and spread out over the North American continent, where the warming weather and the emergence of new leaves provides vast quantities of insects. When summer ends and autumn comes, the declining supply of insects drives them south again. The nonmigratory species that remain are limited by harsh winters and food that is much harder to find.

To live in both the temperate and tropical worlds and to find sufficient food during their long and often intercontinental flights, migrants must be flexible opportunists. Some territorial species that consume nothing but insects during the breeding season, such as orioles and kingbirds, are in winter -- where most of us never see them -- gregarious consumers of fruit and nectar. In the twice-a- year transformation they switch from carnivores to vegetarians.

Such adaptability should place migratory songbirds among the least vulnerable of all animals to the major changes in land-use and wildlife habitat that accompany human activities. To some degree this seems to be the case, for most of them have global populations that are estimated in the millions or tens of millions. Some especially abundant species, such as the Red-eyed Vireo and Blackpoll Warbler, may number in the hundreds of millions.

During the spring and fall migratory periods, migrants concentrate in huge numbers in tree-covered lots and city parks where, for a few weeks, the diversity of birds can rival that found in tropical forests. It is in part this spectacle of abundance, plus their phenomenal ability to navigate at night -- using the stars and the Earth's magnetic field -- that makes migratory songbirds so intriguing to bird watchers and the public at large. Watching large flocks of dozens of species of migratory songbirds pass through the backyard of my own home in Pittsburgh solidified my own lifelong interest in birds.

Saving Songbirds

Despite the apparent abundance of migratory songbirds, they have recently become the focus of the largest conservation effort ever undertaken for a segment of wildlife that is neither exploited -- like ducks -- nor specifically endangered. Among the major efforts to ensure their conservation was the initiation of an extensive program by the National Fish and Wildlife Foundation called Partners in Flight. This undertaking, initiated in 1991, involved many federal and state land management agencies, such as the U.S. Forest Service, Bureau of Land Management, U.S. Fish and Wildlife Service, and the U.S. Geological Survey, as well as many private and non- governmental organizations, including the Audubon Society and the Nature Conservancy. Each year, Partners in Flight sponsors dozens of meetings that bring together scientists, managers, and policymakers; it has also provided millions of dollars to fund vital research and conservation efforts.

Many land management plans in North America now include provisions to protect migratory songbirds, including virtually every federal and most state plans. Many federal, state, and local agencies have changed the way they manage land to allow for the needs of songbirds, and they support research to develop methods for maintaining healthy populations. In an effort to provide funding for the conservation of migratory birds and other wildlife, a broad coalition is now forming to seek legislation that would tax outdoor recreational equipment, including binoculars and camping gear. Such conservation measures have become a vital part of global efforts to protect biodiversity in general.

Why have migratory songbirds generated so much concern? An immediate answer can be found in the widespread publicity given in the 1970s and 1980s to an apparent drop in their number, as counted in local surveys. Declines in migratory birds of all kinds are of particular concern to the millions of Americans who watch birds recreationally, and songbirds enjoy one of the largest constituencies of any type of wildlife. The declines seemed particularly serious for they were interpreted by many people as an indication of broader environmental stress on the scale of continents and even hemispheres.

An Early Alarm

The first hard evidence for a drop in the number of songbirds came from counts that are made each year in suburban parks and in small wooded areas, or woodlots. A number of these early assessments revealed precipitous declines of Neotropical migrants such as warblers, vireos, flycatchers, and thrushes. In many local settings, Neotropical migrants such as Red-eyed Vireos, Hooded Warblers, and Ovenbirds had dropped by 50 percent or more in the span of several decades. On the other hand, the year-round residents, such as chickadees, and the short-distance migrants, such as Northern Cardinals, showed stable or increasing populations.

At about the same time, researchers noticed that many Neotropical migrants were missing from forested patches of up to 500 acres, even when the tree-covered area afforded appropriate habitat and was large enough to satisfy the territorial claims of many pairs of birds. It seemed clear that many migratory songbirds were rejecting these smaller tracts for more continuous expanses of forest.

Because smaller woodlots are the only remaining forest habitats in the vicinity of urban areas, particularly in the mid-Atlantic states, migratory songbirds were disappearing most rapidly from the wooded areas that are most frequently visited by the public. Added impetus came from the fact that among the affected areas were many wooded parks in the suburbs of Washington, D.C.: areas that were once enlivened by the songs of Wood Thrushes and other birds had become mysteriously silent. Migrant songbirds seemed to have left the places where people most watched for them, despite the later evidence, in many cases, of stability in their overall number, nationwide.

A second reason why Neotropical migrants attracted so much attention was the fact that concern over tropical deforestation reached its peak during the time when diminishing numbers of songbirds were also noted. It was only logical to make the connection between deforestation in the tropics and declining songbird populations, since the steepest declines were found in the populations of Neotropical migrants -- whose homes, in winter, were the very regions of Central and South America where trees were being felled.

Evidence from the Breeding Bird Survey

The initial signs of change came from local observations in a limited number of smaller areas. A more powerful and inclusive tool is available to North American ornithologists in the Breeding Bird Survey (BBS), an organized, continent-wide census that was begun in 1966. The field work of the BBS is performed by volunteers, who count songbirds along about three thousand survey routes in the U.S. and Canada. To do this, the census takers drive along coordinated search lines, stopping each half mile for three minutes to tally all the birds that they can see or hear.

The BBS cannot hope to count every bird, everywhere. But because birds sing, the method is efficient and cost-effective, and it provides an indication, although hotly debated, of continental trends in songbird populations. Like political polls and other sampled censuses, the BBS has known limitations, but there is no other comparable tool for monitoring trends in the numbers of any other group of living things -- other than people themselves. The extensive geographic scale of the census, plus the fact that it has been conducted each year for thirty years, has led to the increasing use of songbird populations as more comprehensive "ecological indicators," to the extent that birds reflect the broader health of a habitat or an ecosystem.

In the first ten to fifteen years, surveys taken by the BBS revealed little evidence of systematic declines in most songbird populations, and in fact, many Neotropical migrants were increasing in number. Soon afterward, however, the picture changed. Analyses of trends during the late 1970s and the 1980s suggested that populations of many species were indeed beginning to drop steeply. These declines, coupled with concurrent reports of a diminishing number of migratory flocks seen on weather radar as migrant songbirds crossed the Gulf of Mexico, helped create the mood of urgency that led to the formation of Partners in Flight.

The annual surveys continue, with mounting evidence of change in certain species (Table 1). But while the BBS is a powerful tool, its findings illustrate the complexities involved in attempting to conserve so large and diverse a group of birds. For every species that is decreasing, there is another that is increasing. For every region in which a species is found to be declining, there may be another that shows the opposite trend. And although three decades of BBS census seem like a long time, how can we distinguish meaningful long-term trends from the fluctuations that arise from drought or other natural changes?

As a result of these difficulties, the BBS data have generated a great deal of controversy among the professionals who endeavor to interpret them in terms of significant trends. The devil is in the details, and much of the disagreement deals with such things as statistical methods, sampling errors, and generalizations regarding overall rates of decline.

General Findings

Nonetheless, most interpreters of BBS census data would agree on the following points:

  • When viewed as a whole, and when averaged over large geographical areas, Neotropical migrant populations have remained generally stable. This seems especially true in the western half of the continent.

  • Some forest-dwelling Neotropical migrants are diminishing steeply in most of the regions in which they are traditionally found. Among these, the Wood Thrush (a bird of the robin family that winters mostly from Mexico to Panama) and the smaller Cerulean Warbler (which winters in the northern and central Andes of South America) are declining the fastest, at average rates of 1 to 3 percent per year over the last thirty years.

  • In certain regions, many or even most forest songbirds are declining, especially in the Adirondacks and in the Great Smoky Mountains.

  • Birds found in recently disturbed habitats, such as abandoned fields, have declined as rapidly or even more rapidly than the diminishing forest-dwelling species. Among these are Eastern Towhees and Prairie Warblers. The pattern varies regionally, however, and some birds of disturbed habitats, including the Chipping Sparrow, have increased substantially.

  • Birds found in grasslands, such as Bobolinks and Meadowlarks, have shown the largest and most consistent decline.

  • Some species, such as Hooded Warblers and Ovenbirds, that might be expected to be declining because of lower reproduction and the loss of Neotropical winter habitat, have remarkably stable populations.

  • Few causes of decline can be unambiguously inferred from population trends without the support of additional field studies.

Probable Reasons for Decline

There are several possible reasons that individually or collectively could explain the trends in North American populations of migrant songbirds. As with most unsolved mysteries, there are a number of likely suspects, and each has its proponents.

Changes in summer breeding habitat

Changes in land-use that systematically reduce or alter the summer breeding habitat of migrant songbirds can be expected to diminish the populations of species that live in the interior of forests and grasslands. These changes can also increase the numbers of bird species that thrive in agricultural or residential areas, such as grackles, robins, starlings, and House Finches.

The loss of summer habitat -- as happens when forests are cut down or when stands of trees or other natural landscapes are converted to housing or cultivation -- will clearly be felt by any form of wildlife that raises its young in these more protected areas. More subtle, and at least as important, is the pervasive trend toward habitat fragmentation : the subdivision of more continuous natural habitats, such as forests, into a patchwork of smaller, more isolated parcels. In its most extreme form, called "insularization," once- continuous habitats are replaced by isolated blocks of forest or grassland surrounded by a sea of different, inhospitable habitat such as farmland or residential areas (Figure 1). But fragmentation also includes less obvious transformations, as when activities such as logging isolate patches of mature forest in a setting of younger trees, or when roads or power lines are constructed across natural grassland.

An inevitable result of fragmentation is an increase in the proportion of what is called edge habitat, defined as a boundary between two regions with vegetation of distinctly different heights, such as a forest and an adjoining field of corn. Edge habitat has traditionally been viewed as beneficial to wildlife, and especially for animals known as habitat generalists -- including many game birds and mammals -- that can thrive in a variety of settings. Recent evidence, however, shows that the creation of more extensive edge habitat can diminish the reproductive success of birds that breed near edges, for reasons that are explained below.

Loss of winter habitat

Systematic reductions in the area of tropical forest and other settings where migrant songbirds spend the winter can as easily deplete their number, by reducing opportunities to find food and avoid predators. Such land-use changes can also enhance the population of species such as the Indigo Bunting that winter in highly disturbed habitats such as farm fields.

Losses of tropical forest have been severe and well documented, especially in Central America where Neotropical migrants winter disproportionately. In general, winter ranges of Neotropical migrants cover a far smaller area than the lands in which they breed, which amplifies the effect of winter habitat losses. Nor is the problem limited to Central and South America, for the systematic conversion of grassland to crops in the southern U.S. may also be responsible for the documented decline of some grassland species that are short- distance migrants, such as Sedge Wrens and Henslow's Sparrows.

Reduction in migratory stopover habitat

Another accelerating land-use trend that could clearly affect migratory birds is our own appetite for beach front property, which is converted to human use, around the world, far more intensively than other real estate. About half of the U.S. population now lives in coastal areas, and by the year 2010 this number will grow to about 127 million people. In the springtime, Neotropical migrants must rest and refuel near the coast before and after trips that carry some of them on a 500- to 600-mile, 15-hour non-stop flight from the Yucatan peninsula, across the dark and open water of the Gulf of Mexico.

There can be little doubt that the rapid development of property along the beaches of Texas, Louisiana, and the Florida Panhandle has resulted in a systematic loss of migratory habitat, in this most important of migratory stopover areas. Similar development along the Eastern seaboard, as at Cape May, south of Atlantic City on Delaware Bay, also threatens vital stopover areas for birds. As most bird watchers know, any depletion of these critical habitats could increase mortality during migration, and the inordinate value of beach front real estate adds ever increasing pressure to the threat. The lost of riverine, wooded vegetation in western desert habitats may also pose a particular threat to migrants that cross inhospitable, semi-desert areas.

Any prudent conservation plan for migratory songbirds should include protection and acquisition of key stopover habitat, especially near beaches. Indeed, the best use of small, isolated woodlots and forest preserves in any part of the country may be to manage them to improve the habitat for migrant birds. The "urban forests" of older suburbs and city parks, for example, are used intensively by migrants. Homeowners who live near large bodies of water and in or near isolated woodlots in the middle of large agricultural areas can help migrant birds by the simple expedient of planting native shrubs and trees, and conserving the ones that are there.

Climate change

It would seem only natural that the welfare and hence the numbers of songbirds, as well as other forms of wildlife, would be affected by changes in global and regional climate, and most particularly by extreme climatic events. During years that are unusually wet or dry, the reproduction of birds can be diminished, and as a result, populations in subsequent years will decline. Such trends should be most apparent during prolonged droughts, which for most species reduce the availability of food and even water. The decline of many migratory birds that breed in summer in Europe, for example, has been attributed to droughts that ravaged the Sahel in northern Africa in recent decades. Prolonged periods that are unusually wet, on the other hand, may bring mixed blessings, with benefits for wetland species, such as waterfowl, and problems for others, such as Ovenbirds, that nest on the ground. Were climate to change by the amount now anticipated due to increased global greenhouse gases, and were the warming accompanied by greater extremes of storms and drought and flooding, we should expect noticeable impacts on the populations of songbirds.

At the same time, most migratory birds are flexible habitat generalists with broad geographic ranges, uncommon freedom of movement, and an ability to colonize newly available habitat. As such, they may rank among the groups of species least likely to be affected by climate change. Some, such as the many warblers that breed in spruce-fir forests, could lose substantial parts of their breeding grounds if acid rain, coupled with probable climate change, continue to alter the high-elevation coniferous forests in the East on which they depend for food and nesting sites.

Pollution

Yet another possible cause of songbird decline is the increasing use, worldwide, of pesticides and other contaminants, that may be causing increased mortality and decreased reproduction of songbirds. Problems from pesticides are particularly likely in the wintering grounds of Central and South American countries, where pesticides such as DDT and DDE are still in common use. Recently, researchers in Argentina discovered that tens of thousands of wintering Swainson's Hawks were being killed each year by a pesticide employed to control grasshoppers. The poisoning of wintering Dickcissels by grain farmers in Venezuela provides another example of how pesticides -- in this case DDT -- can affect bird populations. The many agrochemicals now used in North America may also have negative consequences for songbird populations.

Other possible causes

A number of other causes, though seemingly minor, could contribute to an overall decline. Domestic cats are killing machines insofar as birds as concerned, and those that are not confined destroy an estimated hundreds of millions of songbirds per year. They may pose a particular threat to communities of birds in agricultural and grassland settings and in woodlots that are fragmented by residential developments. Towers and the windows of taller buildings are also the cause of death of hundreds of millions of migrating songbirds each year. As we build more radio towers, skyscrapers, and the now-popular commercial buildings that are sheathed in reflective glass, the toll on some species may be sufficient to cause declines. The hunting of songbirds is not a problem for Neotropical migrants, but millions of birds are caught in nets in southern Europe and North Africa where they are used for human food. Any transport of this technology to Central America could pose a severe threat.

Which is most important?

The amount of evidence favoring any of these possible explanations varies greatly. To date, we know most about effects of the loss and fragmentation of summer breeding habitat, and in my view it is the best documented cause of changes in songbird populations. It is also the problem that we can address most readily in our own country, and for these reasons I shall review what is known of this one cause in more detail below. We must keep in mind, however, that when we look ahead, diminishing tropical habitat may pose the greatest threat to migratory songbirds.

The causes I have mentioned are not mutually exclusive, and some species may now be affected by all of them at different stages of their life cycles. Indeed, the multiple factors that can affect migratory songbird populations are what make them such challenging targets for conservation planning.

Summer Breeding Habitat: A Closer Look

Evidence from forest settings

The break-up and replacement of forests by agricultural fields, roads, and suburban tracts are clearly correlated with reductions in the nesting success of many forest species. The connection has been documented in field studies made at both local and regional levels: on the smaller scale, within a woodlot as a function of distance from its edges and on the larger scale, among landscapes that vary in their amount of forest cover.

Landscape fragmentation has so strong an effect on nesting success that birds that breed within and near the edges of small agricultural woodlots may not be producing enough young to replace the adults that die each year. Such regions act as sinks in terms of the population of a given species. The continued presence of songbirds in these areas can be sustained only by immigration from places that are not so affected -- the sources, which are in the interior of large habitat tracts, far from the edge, where a surplus of young is being produced. As landscapes become more and more fragmented, the sink habitats outweigh the sources, and populations decline overall.

The problem is exacerbated for birds that prefer to nest near the edges of woodlots or fields, as these sites can be particularly hazardous. Among those that are naturally drawn to these potential "ecological traps" are Wood Thrushes, Hooded Warblers, and Summer Tanagers. More and more edges are an inevitable product of our tendency to break up larger natural areas into ever smaller ones, and the edges that we create -- such as those that bound agricultural openings -- are by design abrupt, and are purposely maintained that way. They differ considerably from the more tapered, natural edges, bordered with shrubs, that provide added cover for nesting birds. Although most field studies of the effects of fragmentation have been limited to forests and forest birds, there is growing evidence that the nesting success of grassland birds is also reduced near edges and in smaller tracts (Table 2 and Figure 2).

How do added edges affect the populations of songbirds?

We need to first realize that fragmented landscapes provide very favorable habitat for many animals, particularly some mammals and other habitat generalists. Human activities over the years have severely reduced the numbers of top predators such as wolves and mountain lions and as a result, many other mammals, such as deer, raccoons, and opossums, now thrive -- as most suburban dwellers will readily attest. Among other animals to whom we give advantage when we break up the natural landscape are the many snakes that prefer to feed along edges, as well as some birds -- including jays, crows, orioles, and game birds -- which are well adapted to foraging there and move readily among habitat patches. Moreover, in suburban habitats, populations of crows, grackles, and jays are further enhanced by bird feeders and the dense shrubs and tall trees favored in most yards.

As far as songbirds are concerned, the threat of thriving populations of habitat generalists of these kinds is that many of them are at least occasional nest predators. Where raccoons, rat snakes, and jays are particularly abundant, nest predation rates can be much higher than in large, unfragmented habitats. In small woodlots overrun with raccoons, for example, ground nesters lose most of their eggs and nestlings to predators, which may explain why so many nesting birds systematically avoid these areas.

The cowbird problem

Another animal that thrives in fragmented habitats and poses an even greater hazard for many forest-dwelling songbirds is the Brown-headed Cowbird. Cowbirds are obligate brood parasites that lay their eggs in the nests other birds and then fly away, leaving their hosts to hatch and raise their young. More than 200 other species are affected. Cowbirds often remove an egg before laying one of their own; moreover, cowbird eggs usually hatch earlier, giving considerable advantage to their nestlings over those of the host birds in competing for food that is brought to the nest. The smaller the host, the greater the cost of parasitism, although cowbirds often compensate in nests of larger birds by laying more than one egg.

Individual cowbirds typically lay twenty to forty eggs in a season and in captivity have laid as many as seventy-seven. Thus, a few cowbirds could parasitize most of the nests in a woodlot. Some bird species escape by nesting in cavities too small for cowbirds to enter, or by rejecting the alien eggs, either by ejecting them or abandoning the contaminated nest. Most migratory forest birds, however, build "open-cup" nests like those of a robin which are easily intruded upon; nor do they reject the eggs the cowbird leaves, regardless of how different they may appear to us. The Wood Thrush, for example, accepts and incubates cowbird eggs, which are white with brown speckles, even though its own are bright blue, like those of a robin.

The lack of compensating defenses on the part of many songbirds makes an increase in the number of cowbirds an especially serious threat. Birds have a long evolutionary history of coping with nest predators such as raccoons or snakes; when eggs or young are taken from their nests, most birds move to another site and quickly try again. The overall result, in the long run, is a systematic departure of songbirds from areas of persistent nest predation.

Those songbirds that raise cowbirds, however, behave as though they had succeeded in their breeding duties and may not nest again that year. Their dedication to young cowbirds left in their care can deplete their numbers as effectively as if they had been made sterile. Thus, areas of chronically high parasitism -- but low nest predation rates -- can act as "ecological traps" that continue to attract birds while draining their regional populations. At least three species or subspecies with small geographic breeding ranges -- the Kirtland's Warbler, Black-capped Vireo, and Least Bell's Vireo, are endangered as a result of the combined effects of cowbird parasitism and nest predation. These species have no refuges from cowbird parasitism and only survive because of intensive efforts to control cowbirds in remaining patches of breeding habitat.

The reason cowbirds thrive when land is fragmented is that they are not tied to the forest. They find their food -- typically insects and seeds -- in open areas of short grass and bare ground, and because they do not have to feed their own young, they are free to commute long distances, of as much as four miles, to get it. Landscapes in which woodlots (where most of them breed) are mixed with farms and pastures (where they eat) are for cowbirds an ideal habitat.

Cowbirds evolved in close association with herds of bison and other hoofed animals whose footsteps kicked up food for them to eat. Moreover, until the European settlement of America cowbirds were largely restricted to the northern Great Plains. When bison herds moved within sight of forest edges the cowbirds commuted between the trees -- where they found nests in which to lay their eggs -- and the wandering herd.

With the coming of pastures and farms, cowbirds expanded their original, more confined range to include all parts of North America where grain and domestic cattle were raised. In the process their populations increased, and songbirds with little history of exposure to the cowbird's ways were introduced to nest parasitism for the first time. As a result, many host songbirds have no evolved resistance to a threat that grows each year. The problem may also have been exacerbated by a greater food supply in winter: cowbirds winter mainly in grain fields in the southern U.S., where mechanical harvesting leaves more grain on the ground than was once the case. Because of these changes, the continental cowbird population is now counted in the hundreds of millions, outnumbering by far the majority of their hosts.

Sources, sinks, and parcel size

Cowbird parasitism is a particularly severe problem in the agricultural Midwest (Table 3) where over 80 percent of the nests of some species are parasitized and where as many as five cowbird eggs can be found in the nests of some of their hosts. Because predation is also severe in many of these landscapes, small agricultural woodlots rank high as probable population sinks.

In contrast, nest parasitism and predation rates are much lower in the larger forests that cover extended parts of the Missouri Ozarks, south central Indiana, and the northern parts of Wisconsin and Minnesota (Figure 3). In the Midwest these areas probably serve as population sources that balance the many small woodlots where there is virtually no production of young birds. In other forest settings, losses near the edges may be compensated by producing a surplus in the interior.

The concept of source and sink areas can be helpful in conserving songbird populations. Conservation efforts will be most effective if they focus on the largest tracts at any scale that still remain in a more natural state. In terms of the continent as a whole, the extensive and regrowing forests of New England, the Appalachians, East-Central Canada, the U.S. Great Lakes region, and the Ozarks may be among the most critical props for sustaining songbird populations. On the smaller, regional scale, the most critical areas are landscapes that are primarily forested, of at least 10,000 acres and preferably 50,000.

Conserving forest birds

Most of the large tracts that remain contain a high proportion of federal, state, and other public lands, which can be managed as a unit. Keeping these large tracts in public ownership and managing them to avoid further fragmentation is the first step in any plan that addresses the plight of migratory songbirds. Were large sections of these public lands turned over to agencies or private landowners who felt no obligation to conserve wildlife, the effects on the populations of some migratory songbirds could be catastrophic. A case study of the potential consequences can be found in the Shawnee National Forest of Southern Illinois. Within its boundaries are more than 260,000 acres of forest land -- about one third the size of Rhode Island -- but numerous private agricultural openings divide the forest into much smaller pieces, such that none of the woodlots is larger than 10,000 acres. In this landscape, cowbird parasitism and nest predation rates are so high that the entire national forest may be a population sink for some bird species.

The conservation of migratory songbirds in heavily settled and intensively used agricultural areas calls for a different approach. In agricultural and urban landscapes, most patches of forest are privately owned and smaller than 10,000 acres. Here the best approach to conservation is to focus on a few of the largest sites available and to work with private landowners to reforest small openings within and adjacent to the forests, making use of conservation easements and tax breaks to compensate landowners for lost income. Parks or other public lands could form the core of areas that can be expanded by the acquisition of land from willing sellers. These medium-sized sites may not be large enough to serve as population sources for some bird species, but enlarging them may reduce the drain by increasing nesting success in these areas, and may even create source habitat for species that are more resistant to habitat fragmentation, such as the Kentucky Warbler and Acadian Flycatcher.

These efforts to conserve songbirds through land planning define a prioritized working list: first preserving the largest remaining areas of chiefly public lands, then devoting attention to restoring and supplementing available plots of medium size, which in the Midwest are on the order of 500 to 10,000 acres.

Left behind in this strategy of triage are the areas closest and dearest to most bird-watchers: the all-too-common woodlots, less than about one square mile in size, that probably no longer serve effectively as summer breeding grounds for migratory songbirds. Conservation strategies for these woodlots are better directed at other kinds of plants or wildlife and at creating habitat for birds that briefly stop there on migration.

The situation in grasslands

For grassland birds such as Bobolinks, Upland Sandpipers, meadowlarks, and many sparrows, fragmentation of the land may not be as damaging as the outright loss of habitat. As with forest birds, those that nest in grasslands prefer larger, more continuous tracts and show some evidence of greater nesting failure in fragmented parcels of land. But they are much less area-sensitive than many forest birds, and many species do better after severe disturbances such as fire and heavy grazing than without them.

Until about 1960 grassland birds thrived in human-dominated landscapes. Species such as Upland Sandpipers that required short grass lived in pastures, and those that needed taller grass, such as Sedge Wrens and Bobolinks, found their homes in hayfields. Since then, however, the farm acreage that is devoted to pasture and hayfields has fallen as the emphasis shifted to row crops like corn and wheat and soybeans. With the availability of faster-growing grass seeds, hayfields are also harvested much earlier than was the case before, destroying many of the eggs and young of grassland birds that attempt to nest there.

One result of these evolving shifts in land-use is very large, continent-wide decreases in the populations of most grassland birds. Over the last few years, some of these trends have reversed as more and more acres have been set aside as part of the Conservation Reserve Program (CRP), which subsidizes farmers not to grow crops on marginal farm land. Although not all fallow fields are suitable for grassland birds, the policy has benefited at least some of them, and particularly those that breed in marshy lands, such as some waterfowl.

The problem has not been permanently solved, however, because future changes in farm policy could threaten the CRP, and set-aside acreage is always likely to be reconverted to row crops when grain prices are high. For this reason, private lands may not be a reliable, long-term source of breeding habitat for grassland birds. Conserving their populations during periods of high grain prices and low support will require a network of grassland preserves. An auspicious example of such a preserve is the 20,000-acre Midewin National Tallgrass Prairie, which was formerly the Joliet Army Ammunition Plant, near Chicago. When the U.S. Forest Service took over the military reservation in 1995, they began an ambitious effort to restore it to native grassland. The project is particularly promising in that it is large enough to minimize adverse edge effects and to provide suitable habitat for both species that require short grass, such as Upland Sandpipers, and others, like Henslow's Sparrows and Sedge Wrens, that prefer taller, denser grasslands.

Shrublands and other disturbed habitats

In general, birds that prefer shrublands can adapt to newly- disturbed areas and they have benefited from human activities. They include, among others, Baltimore Orioles, Prairie Warblers, Blue- winged Warblers, Eastern Towhees, Gray Catbirds, and Brown Thrashers. As forests are cleared, vast amounts of shrubby "edge" and "old field" habitats have been created. Clearcutting -- the predominant method of harvesting wood in most forests -- also generates habitat for shrubland birds in the years immediately after an area is cut. Game management and even residential development can create shrubby habitat for at least some species of birds.

As a result of these practices, there have been enormous increases in populations of many shrubland birds, and for this reason, conservationists have understandably paid them less attention. Nevertheless, some of the fastest-declining species in North America are found among this group, including the Prairie and Golden-winged warblers, Yellow-breasted Chat, and the Eastern Towhee.

Most of the declines of these and other species can be traced to more specific, regional changes in land-use patterns. Shrubland habitats have disappeared in New England in the last few decades as abandoned farms have reverted to forest cover. In the Midwest, shrubby areas have disappeared as intensive road-to-road row crops have replaced the more diverse agricultural landscapes of hedgerows and wooded stream corridors that were favored by agricultural practices through the 1950s. Some of the population declines simply reflect the retreat of species from areas that they had never inhabited until humans arrived.

The gains and losses of shrubland birds with land-use change are most often local, and sometimes temporary, and may tend to balance in the long run. A problem arises, however, in the group of species that have been deprived of habitat not by more environmental disturbance, but by less.

Before the arrival of European settlers, birds such as Bell's Vireos and Bachman's Sparrows made their nests in the early successional habitats that followed floods and natural fires. Today, most of our rivers are clad in levees, and forests and grasslands are protected by strenuous efforts to prevent fires altogether. Faced with far fewer natural disturbances, many shrubland species now depend upon human disruption of the land for their breeding grounds, such as mining and road-building and clear-cutting. Just as changes in farm policy call the tune for grassland birds, changes in logging practices and land management in general will ultimately dictate population trends of shrubland birds.

Combining game management with the conservation of declining shrubland birds is a particularly promising approach. The habitat requirements of these two groups of species -- that include in the first case pheasants and quail and in the second, Field Sparrows and Prairie Warblers -- are sufficiently similar that policies to preserve shrubby fields and edges may benefit both of them. Similarly, commercial forests that employ clearcutting as a silvicultural tool can manage recently cut lands for the benefit of both game and songbirds.

Other Causes of Decline: Loss of Winter Habitat

Some ornithologists feel that the major cause of declines in North American songbirds is the loss of tropical forests, and there is little doubt that certain species, such as the Wood Thrush and Cerulean Warbler, have been severely affected by a loss of winter habitat in these regions. Conservation of lowland forests in the West Indies, southern Central America, and the lower foothills of the Andes is especially critical for halting some declines. As the amount of tropical forest continues to shrink, the populations of some species of songbirds that winter chiefly in forest habitats will drop, despite efforts taken here, in temperate North America.

At the same time, most Neotropical migrants are quite flexible in their choice of winter habitat, and can occupy regions that are only marginally used by the forest birds that breed in the tropics. And although migrant birds fit into a smaller geographical range in winter, their needs are fewer. Freed from the constraints of breeding that tie them down in summer, they need only find sufficient food for the day, and avoid predation. Disturbed habitats, such as pastures with scattered trees and those coffee plantations that grow coffee shrubs in the shade of tall trees, are heavily used by migratory songbirds for these purposes.

If they are indeed more flexible in their choice of winter habitat, Neotropical migrants may be less sensitive to what goes on in pristine tropical forests than to the quality of habitat available to them in the more disturbed landscapes of the Neotropics, such as pastures, agricultural fields, coffee plantations, and residential areas. In this case, the best way to sustain populations of many migrant songbirds in the tropics may be to encourage the development of shaded plantations, as opposed to other methods of coffee cultivation, and the preservation of wooded stream corridors in agricultural landscapes, which would also help water quality. Tropical forests need to be conserved, but perhaps more to meet the habitat requirements of the year-round residents that nest there, than for winter migrants.

Losses of grassland habitat in the southern U.S. may be responsible for some of the declines of grassland species, and particularly the short-distance migrants that spend their winters on this side of the Gulf of Mexico. Coastal grasslands have systematically been replaced by rice and other row crops, and introduced fire ants may also have reduced habitat suitability in many areas. Preserving the grasslands where these bird species winter may be every bit as important as restoring the areas where they breed. Also, losses of grassland as far away as southern South America may have contributed to the decline of at least one species of North American songbirds: the Bobolink, that winters there. We know even less about what limits the populations of grassland birds than we do about their forest- dwelling cousins.

Conclusion

Conserving migratory songbirds, however popular the cause, can be a very difficult task, for nearly any human activity will affect different species in different ways: some for better, some for worse. Almost any land that is taken for urban, agricultural, silvicultural, or other human use comes at the expense of songbirds of one species or another. The gradual conversion of continuous cover, such as forests or prairies, into patchworks of other uses has a direct impact on songbirds. These facts apply to either of the two hemispheres in which migrant birds spend their lives.

Clearly, we cannot just say no to agriculture or forestry or even habitat fragmentation. We can, however, be more aware of what is gained and what is lost when these activities are undertaken, and take steps, as best we can, that minimize the harmful impacts on songbird populations. To do this wisely will require a better understanding of the natural variability and resilience of their numbers overall, a deeper knowledge of the needs and current plight of individual species, and much clearer answers to what lies behind their present-day declines.

The long-term preservation of migratory songbirds of most kinds will depend primarily upon maintaining and restoring a network of large reserves where better breeding opportunities can balance what happens in more fragmented habitats. Only if we preserve these areas that produce a surplus can we expect the continued presence of most songbirds in our own neighborhoods. We shall also need to preserve and restore the stopover areas through which songbirds migrate, and lend help, where we can, in the preservation of their winter habitat.

These strategies will solve some but not all of the problems. Uncontrolled deer populations, for example, may destroy habitat for some bird species by removing understory vegetation. Bird populations in shrubland, grassland, and agricultural landscapes will continue to vary with changes in crop prices, farm policy, and wildlife management practices, and maintaining the populations of these birds will require an ongoing effort on the part of preserve managers and policymakers. If winter habitat losses continue and if tropical countries adopt the intensive agricultural practices employed by North American farmers, some bird species that we think of as our own will inevitably decline, regardless of what we do in North America -- just as what we do or don't do in the cause of conservation affect others who live in Central and South America.

Global changes brought about by human activities affect all living creatures, and songbirds have become the most visible indicators of the consequences of these changes. Songbirds serve as a kind of barometer of the general state of the environment and a ready reminder of the underlying need for conservation and biodiversity. People see birds, count them, and care about them. What is good for birds bodes well for other animals, including us. Songbirds link conservation efforts on different continents and command economic attention through ecotourism and their undeniable benefits for the forest products industry. Perhaps better than any other living thing, migratory songbirds illustrate the interconnectedness of the planet on which we live. Their regular arrival in the spring and departure in the fall reminds us of their transience, and of the ties that bind our needs to those of other lands.


Reviewers

Prof. Robert Askins is an ecologist in the Department of Zoology at Connecticut College in New London, who studies the ecology of migratory birds in both their breeding habitats in New England and their winter habitats in the U.S. Virgin Islands.

Prof. Francesca Cuthbert is an ornithologist in the Department of Fisheries and Wildlife at the University of Minnesota in St. Paul, and in summer a visiting Professor at the University of Michigan Biological Station at Pellston in northern Michigan. She specializes in the conservation of avian biodiversity.

Dr. David Wilcove, whose research focuses on island biogeography and the conservation of endangered species, is a senior ecologist in the Washington, D.C. office of the Environmental Defense Fund. The EDF is a national non-profit organization, based in New York, that links science, economics, and law to create innovative, economically-viable solutions to today's environmental problems.

Scientific reviewers provide technical advice to the authors and Editor, who bear ultimate responsibility for the accuracy and balance of any opinions that are expressed.

For Further Reading

Where Have All the Songbirds Gone? by John Terborgh. Princeton Univ. Press, Princeton, N.J. 207pp, 1989.

The Breeding Bird Survey home page on the World Wide Web.

"Nest Losses, Nest Gains" by Scott Robinson, Natural History Magazine, vol 105 (no. 7), pp 40-47, July, 1996.


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