A Perilous FLIGHT.
The island is known for its huge diversity of gulls, sandpipers, and terns. But it is also a popular place for migrating forest songbirds, which find sanctuary within its few acres of trees on their trip north. "Sometimes, the woods are full of warblers," said Gene Scarpulla, of the Maryland Ornithological Society, who monitors birds on the island. "It's a stopover place for them."
Once, some of these birds ended their long journeys just a few miles away, on the mainland. Now, few bother. Scarpulla's records show island sightings of many forest-dwellers, such as ovenbird, orchard oriole, hooded warbler, and others, that will stop for a break on Hart-Miller but wouldn't think of building a nest across the water in the rapidly developing Baltimore Washington corridor.
Wooded areas that surrounded the Baltimore-Washington region only a few decades ago have been cut down, paved over, or chopped into ever-smaller pieces to make way for homes, malls, and highways. The large blocks of forests that once offered nesting sites for birds now spell peril as they are overrun by cats, blue jays, and other predators attracted by the changing habitat. It is, in the words of one ornithologist, a "black hole" for forest-nesting birds.
A recent Regional Ecosystem Analysis by AMERICAN FORESTS illustrates why. Using satellite images to measure land-use changes, it showed that overall tree cover in the Baltimore-Washington area declined from 51 percent to 39 percent between 1973 and 1997. From a forest bird's perspective, things are worse. The analysis revealed that heavily forested areas--places with more than 50 percent tree cover--declined at an even faster rate, from 820,569 acres to 555,090, a 32 percent drop. Not only is the region losing trees overall, but blocks of woods are becoming smaller and increasingly isolated.
Nor are concerns limited to the Baltimore-Washington corridor. For nearly two decades, many birders have worried about the health of migratory bird populations inhabiting the nation's forests. North America has about 250 species of "neotropical migrants," birds that winter in the tropics but fly huge distances to more temperate climates to build nests and rear young. About two-thirds of those long-distance migrants are forest-dwelling birds.
These birds move not because of temperature but for food. They go north to harvest the crop of worms and bugs from the forests each spring, food they will use to rear their young. They retreat south as food becomes scarce. It is a mutually beneficial relationship for birds and forests: Some have estimated the forest-pest control value of birds at $5,000 an acre.
But no meal is free, and birds pay a high price for their forest banquet. Migration, always perilous, is becoming more hazardous all the time. The migration path for most eastern birds hugs the Atlantic coast, one of the most rapidly developing parts of the country. Along the route, new buildings, communication towers, and other flight hazards are going up in front of the birds, which migrate mostly at night. Safe wooded stopover sites, where they can rest and refuel before continuing their flight, are increasingly scarce. That's one of the reasons AMERICAN FORESTS has funded a local conservation group, the Alliance for the Chesapeake Bay, to expand the Hart-Miller Island forest by planting an additional 20,000 trees (see sidebar, page 37).
Worries don't end with the migration. Scientists are finding danger signs in forest destinations, Alarm bells began going off in the late 1980s when some studies suggested that forest-dwelling neotropical migrants were in widespread decline. As woodlands and neighborhoods lost their warblers and tanagers, some even warned of a new "silent spring"--a reference to Rachel Carson's groundbreaking book on the dangers of pesticides and insecticides.
New research suggests those early warnings were overstated. Red flags initially flew in large part because of the public perception around places such as the Washington, DC, region that backyards and small parks had grown strangely silent. And, indeed, surveys show those places are remarkably barren of birds. Many urban and suburban areas are so filled with hazards and predators it's unlikely any nesting bird will successfully rear young. But a decade of analysis has resulted in a much different, albeit murkier, overall picture for forest birds. In fact, most forest bird populations are in good shape. (As a group, grassland-dwelling birds tend to be worse off--it appears nothing is less secure today than a piece of idle land.)
But all is not well in the woods. While overall populations of many forest dwelling species are stable or increasing, several have suffered serious declines. In the past quarter century, the North American Breeding Bird Survey--the main source for bird population trends--has documented a 51 percent decline in the cerulean warbler, a 48 percent decline in the olive-sided flycatcher, a 41 percent drop in the wood thrush and a 34 percent drop in the Eastern wood-pewee.
But the story varies by species and by place. Although the East Coast and Midwest have suffered forest bird losses, the Appalachian forests and Western mountains have generally maintained or increased populations. Some populations that are stable overall have undergone significant regional declines. The scarlet tanager, for instance, has a stable population, although it has suffered significant declines along the Atlantic Coast from Washington to Boston.
At first glance, you might wonder why any forest songbird should be in jeopardy. After all, the entire eastern United States was heavily logged during the 1800s to supply the growing nation's need for fuel, fencing, and other products. In 1900 alone, it's estimated that 15 to 20 million acres of forests were consumed to meet the demand for railroad ties. A few birds did disappear: the ivorybilled woodpecker, which is thought to be extinct, and the Carolina parakeet and Bachman's warbler. Any species that survived has shown resilience in the face of massive habitat change.
The decline of birds in some areas, scientists worry, could be a sign of a more insidious change in the forest landscape. When loggers moved across the East and Midwest a century ago, they left a devastated landscape in their wake. Much of that became agriculture, but large chunks regrew into new forests.
"None of those big forest patches ever winked out completely," says Chris Haney, a biologist with the Wilderness Society.
Today, the eastern United States has more forest than it had a century ago, but those forests aren't a solid blanket covering the landscape. They are pockmarked by subdivisions, farms, and clearcuts; what's left is sliced by roads as well as power line and pipeline right-of-ways. "Now," Haney says, "we are incrementally degrading every place."
As forests become chopped up into ever smaller chunks, the amount of "edge" habitat increases for predators such as blue jays, crows, raccoons, opossums, feral cats, black rat snakes, and others that prey upon birds or their eggs. Many of those predators are not found deep within forests but may venture several hundred feet into the woods. As tracts become small enough, the entire woodlot can be overrun. It's not just the size, but also the shape of the forest that is important. Big forest patches that are long corridors do not offer as much secure interior habitat as a circular or square forest.
A particular menace is the brown-headed catbird. Historically a prairie-dwelling bird, it followed herds of buffalo, eating worms kicked up by their hooves. In recent decades, cowbirds have thrived by moving into the forest-fragmented landscape of the Midwest and East where plows and cows turn up meals.
Cowbirds don't build nests. The females lay eggs in other birds' nests, then abandon them. Some birds, such as blue jays, recognize cowbird eggs and throw them out. Many forest-dwelling songbirds never had to develop such defensive tactics. When cowbirds penetrate the forest edge and leave eggs, the migratory birds raise them-often at the expense of their own young.
In the agricultural Midwest, more than 80 percent of the nests of some species--veery, wood thrush, hooded warbler, red-eyed vireo, scarlet tanager, and others--host cowbird eggs. The cowbird has helped drive the Kirtland's warbler, which breeds in Michigan's pine forests, to the brink of extinction. Saving the warbler has meant a massive, ongoing campaign to trap and remove cowbirds.
Such impacts disproportionately affect long-distance migratory birds. They typically get a chance to breed only once a year, while natives or short-distance migrants may lay several batches of eggs. If a warbler's eggs fall victim to a predator, or are replaced by a cowbird's, they won't get a chance to produce another batch until the next year.
Studies have shown that predation and cowbird parasitism decreases as nests are located farther away from edges and toward forest interiors. But how big is big enough to protect what are known as "area sensitive" forest birds?
The answer is far from simple, and research has uncovered starkly different answers. Scott Robinson, an ornithologist with the University of Illinois, has suggested that the entire 260,000-acre Shawnee National Forest in southern Illinois is so fragmented by numerous private agricultural openings that the entire forest could be a population "sink" for some bird species.
The Cornell Lab of Ornithology several years ago launched "Project Tanager" to study the forest habitat needs of scarlet, summer, and western tanagers. Tanagers are sensitive to fragmentation; managing for them should help protect dozens of other birds that use similar habitats. But the results showed that when it comes to fragmentation, there is not a one-size-fits-all answer, even for a single species.
How much forest a tanager needs depends on the surrounding landscape. A 20-acre woodland surrounded by fields is not likely to attract tanagers. But put the same woodland near a large area of forest--2,500 acres or more--and it's likely to have tanagers. The farther a woodlot is from a larger forest, the bigger it must be to attract tanagers.
"Not all fragmentation is equal," says Ken Rosenberg, the lab's assistant director of conservation. "As you start dropping below 70 percent forest cover for the larger region, then the size of the forest block starts becoming much more important. The overall amount of forest that's left in the large region seems to play an overriding role in what happens at smaller scales,"
Large areas produce big numbers of birds, which spill over into smaller, usually less productive, surrounding woods. As the amount of woods declines and the distance between them increases, there are fewer birds to disperse into the surrounding areas.
There are other shades to the fragmentation picture as well. Fragmentation by forestry activities tends to be less harmful than fragmentation by agriculture. Fragmentation by development is more harmful still. Worse, it's also irreversible. "We're probably never going to be in a situation where we bulldoze shopping malls and plant trees," Rosenberg says. "It is that most permanent loss of habitat that is most egregious, the most critical loss that we're facing. And I don't know what to do about reversing that."
Even bringing back forests on former agricultural sites is no easy task. Left alone, a field may be overrun by exotic species such as honeysuckle, which can outcompete seeds of native trees. In some regions, browsing deer will halt forest regrowth before it starts. Restoring means not only planting the right mix of trees, but revisiting the site year after year, replacing dead trees, until the effort takes hold.
"The price of reforestation is heinously expensive on a per-acre basis," says John Graham, lands steward for The Nature Conservancy's Milford Neck Preserve along Delaware Bay, another Global ReLeaf Forest site.
The site is particularly important for many birds that hug the coast during migration, looking for a safe place to rest. Rather than start from scratch, the Conservancy is using AMERICAN FORESTS' grant money to help it create a solid forest patch of more than 1,000 acres by planting just 80 acres of trees. It's doing that by filling in agricultural fields that previously broke up the forest, forming huge amounts of "edge" habitat.
"We're dealing with relatively small patches of farmland, which makes it economically feasible," Graham says. "When we fill these smallish patches in. we will have a very significant block of contiguous forest out there, with much less edge."
Because the job of restoration is so monumental, much of the bird conservation focus is on making sure large chunks of habitat are maintained to begin with. Large chunks not only provide a lot of secure forest interior, but are also more apt to contain a mix of forest ages and types, each favoring a particular species.
To help make that happen, the National Fish and Wildlife Foundation launched Partners In Flight a decade ago. Working internationally with government officials, scientists, and forest-owning companies, Partners in Flight supports research and planning activities that will help bird conservation decisionmaking for decades to come, both here and on tropical wintering grounds, which are also under pressure.
What's at stake is not so much a matter of heading off extinction, but "keeping common birds common," the theme for Partners in Flight. After all, the forest birds that remain today have been around for thousands of years--they have seen glaciers come and go; they have withstood widespread deforestation. Even birds that scientists are most worried about, such as the wood thrush, number in the millions. If large forest tracks end up becoming second-home developments, many birds will likely survive. But their songs will become more distant as they retreat farther into whatever forest patches remain. AF
Karl Blankenship is editor of the Alliance for the Chesapeake Bay's Bay Journal.
CREATING A PLACE FOR BIRDS TO CALL HOME
Sprawl is eliminating many of the forested areas migrating songbirds depend on in their journeys to and from their southern wintering grounds. A number of AMERICAN FORESTS' 2000 projects are aimed at restoring degraded or cleared farmland or creating wooded corridors for birds to use for stopover points or breeding sites. To contribute to any of these projects, call 800/545-TREE (8733) or visit www.americanforests.org.
California: Afton Canyon
Planting 10,000 willow and cottonwood trees along 400 acres of degraded riparian habitat along California's Mojave River. Before trees can be planted, invasive saltcedar must be removed. The Afton Canyon project is ongoing and AMERICAN FORESTS has so far planted trees in a 700-acre area.
Delaware: Milford Neck Preserve
Restoring a coastal plain forest along Delaware Bay on land owned by The Nature Conservancy. Over three years, approximately 33,000 seedlings will improve habitat for forest-interior dwelling birds and other wildlife.
Georgia: Okefenokee National Wildlife Refuge
In an effort to reestablish and expand endangered longleaf pine in the refuge, the US Fish and Wildlife Service will plant 10,000 seedlings on the refuge's 33,000 acres of upland forest. The trees will be planted in small natural openings and areas ravaged by fires or logging.
Louisiana: Bayou Cocodrie National Wildlife Refuge
The migration routes, stopover points, and breeding habitat of neotropical songbirds in Louisiana will benefit from the planting of 150,000 native bottomland hardwoods on the Bayou Cocodrie National Wildlife Refuge. The plantings also will provide breeding habitat and wintering grounds for waterfowl and habitat for the threatened Louisiana black bear.
Maryland: Hart-Miller Island
This state-owned island, a repository for dredge material from Baltimore Harbor, seems an unlikely place for migrating songbirds, but it is in fact an important stopover point along the birds' route. The Alliance for the Chesapeake Bay is planting 20,000 trees on the island in a two-year project.
Mississippi: St. Catherine Creek National Wildlife Refuge
Restoring native bottomland hardwoods to these former soybean fields will provide migrating, breeding, and stopover sites for songbirds. Other animals that will benefit: Louisiana black bear, American bald eagle, peregrine falcon, and wintering and breeding waterfowl.
Texas: Red River County
This project will restore 737 acres of degraded wetlands and adjacent wildlife habitat by planting 147,400 native trees along two miles of the Red River that were previously drained and farmed. The owner of the property has put it into a conservation easement.
Texas: San Antonio Missions Hike & Bike Trail
This project will plant 20,000 trees along the eight-mile recreational trail connecting the Alamo River Walk and the San Antonio missions. The cooling canopy the trees will provide will create an important path for bird migration through the city.
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|Date:||Sep 22, 2000|
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