Читать книгу Mountains and Marshes - David Rains Wallace - Страница 15

a stop on the flyway

Oakland’s Lake Merritt is, oddly enough, the oldest migratory bird refuge in the United States, established by the state in 1870 as a sanctuary for wintering ducks and other waterbirds. This was mainly for the benefit of the nouveau riche residents of mansions that ringed the lake after entrepreneur Samuel Merritt created it by damming a polluted tidal inlet to flush out the sewage. The residents probably liked having ducks to admire at the ends of their lawns: they certainly didn’t want shotguns blasting away at them where their children and dogs were playing. Still, the little lake protected waterfowl during times when hunters thought nothing of killing hundreds a day, and it later played a surprising role in migratory bird science.

Bird migration is one of the great challenges to human understanding. Early records like the Bible mention it, but the “why,” “how,” and “where” of it remained mysteries. Classical Greek philosophers knew more about math and physics. Aristotle thought redstarts, summer migrants in Europe, turned into redbreasts in winter. (Old World redstarts and redbreasts are songbirds that resemble New World bluebirds, although they aren’t blue.) The notion seems fanciful, but it was probably based on observation. Birds molt and change plumage before migrating, and species can be hard to distinguish then. Redstarts and redbreasts look similar anyway, so when redbreasts were present in winter, it made sense to see them as winter versions of redstarts. The redstart’s winter home in Africa was unknown.

Two millennia later, the English essayist Robert Burton was as puzzled as the Greeks: “In winter not a bird is in Muskovy to be found,” he writes in his 1621 Anatomy of Melancholy,

but at the spring in an instant the woods and hedges are full of them, saith Herbastein. How comes it to pass? Do they sleep in winter like Gesner’s Alpine mice; or do they lie hid, as Olaus affirms, in the bottoms of lakes and rivers, holding their breath? . . . Or do they follow the sun, as Peter Martyr manifestly convicts out of his own knowledge; for when he was ambassador in Egypt, he saw swallows, Spanish kites, and many such European birds in December and January. . . . Or do they lie hid in caves, rocks and hollow trees, as most think, in deep tin mines or sea cliffs, as Mr. Carew gives out?

Burton retained something of the medieval tradition of relying more on scholarship than personal observation for information. But two centuries later, Gilbert White, a close observer whose The Natural History of Selborne initiated today’s nature writing tradition, remained unsure about many birds’ seasonal disappearances from the Hampshire countryside, which he had studied all his life. Geographic exploration had shown that birds departing in the fall went south and that others came from farther north at the same time. But White still found it hard to believe that little finches, thrushes, and flycatchers could fly all the way to Africa. He also could not confidently reject the traditional belief that swallows and swifts dived into lakes and ponds to spend the winter dormant in mud. Their behavior contributed to this idea, since they spend a lot of time swooping over water to catch insects.

Technology solved those problems. Researchers put radio transmitters on white-fronted geese in California, follow them to Yukon Delta breeding grounds, then to wintering grounds in northwestern Mexico. But the “why” and “how” of migration remain problematic. We know that billions of birds move out of the tropics every spring, seeking the vast food and space resources of northern summer. We know almost nothing about the phenomenon’s evolution.

Ornithologists once thought, for example, that showy songbirds like the orioles, tanagers, buntings, and warblers that nest here in summer evolved in North America and started migrating to the tropics when ice age cooling forced them south. But studies in the tropics found many resident species of our gaudiest summer migrants—orioles, tanagers, and buntings—and many are gaudier than ours. While some wood warbler species probably started migrating south because of global cooling, most of our summer songbirds probably moved north from the tropics as climate warmed. Two wood warbler species, the yellow-rumped and Townsend’s, winter here, but the orioles, tanagers, and buntings all go south.

Each avian group has evolved its migration patterns differently, and fragile-boned birds leave few fossils. There are, fortunately, still plenty of living ones to study. Yet the results are bewilderingly complex. Various birds seem to use every migration method imaginable short of compass and sextant. Almost all migrants probably steer by coastlines, mountain ranges, and other landmarks to some degree. Social birds such as geese and cranes have traditional routes that they teach their young. On the other hand, many fledgling birds make their first migration independently, implying a genetic programming that prompts them to fly in the right direction for the right distance. (Sometimes they fly in the wrong direction, which is why the Bay Area regularly gets a few eastern or Old World birds.)

Night-flying migrants, of which there are many, may steer by the stars; day-flying ones by the sun or, on cloudy days, by polarized sunlight. Pigeons and doves have tiny deposits of magnetic crystals in their heads that allow them to orient themselves by earth’s magnetic field. Some birds may hear low-frequency sounds like surf over long distances, which could help them follow coastlines or make landfalls when they are flying at twenty thousand feet at night, as many do.

Migratory birds have been a spectacular aspect of American history since Europeans arrived, if a sad one. Colonial naturalists, used to decimated Old World faunas, couldn’t believe the abundance and diversity of the birds they found in eastern forests and wetlands, but it took only a few centuries to make those qualities, in fact, unbelievable. It is hard now to imagine eastern forests with clouds of passenger pigeons and flocks of Carolina parakeets. Avian faunas were less spectacular as the frontier moved through the arid West, but when Europeans reached the Bay Area, they found huge migratory bird concentrations. Sir Francis Drake provisioned his ship for crossing the Pacific with seabirds and mammals from the Farallon Islands.

As elsewhere, colonists quickly reduced those concentrations. Market hunting became a ruthlessly efficient industry and remained so into the early twentieth century. By the 1820s, Russian fur traders were killing fifty thousand western gulls a year on Farallon breeding islands for meat and feathers. Even the fishy-tasting eggs of seabirds such as murres from the Farallones became a profitable commodity in the days before chicken farms. An estimated four hundred thousand common murres originally nested on the islands; by 1900, “egging” had reduced the population to about twenty thousand.

In the long run, however, conversion of habitat, particularly Bay marshes, into farmland, salt ponds, and towns had a greater overall impact. It’s hard to say what bird species habitat loss may have extirpated from the area, since little is known about what was here and the loss happened so fast. At least one disappearance is well-known, that of the yellow-billed cuckoo, a South American migrant that bred in Bay Area riparian woodlands into the nineteenth century. Civilization’s thirst for irrigation water had expunged it by the twentieth.

Considering San Francisco Bay’s importance as one of the world’s major estuarine harbors, the late-nineteenth-century’s conservation movement might have championed its bird habitats, but the Bay’s economic potential outweighed this. John Muir may have worried about ducks and shorebirds as he managed his fruit ranch in Martinez in the 1880s and ’90s, but there wasn’t much he could do about Bay wetlands, as opposed to remote Yosemite. (And the city even got part of that in Hetch Hetchy Reservoir.) In 1909, the United States designated some of the Farallon Islands as a national wildlife refuge, but aside from kicking out the market hunters, it did little to protect wildlife. Through the 1920s, as early federal refuges dotted more rural regions, the Bay Area maintained the boomer stance typical of the forty-niner past. Entrepreneurs had plans for the Bay itself—filling it for real estate—and its waterfowl habitat didn’t get even a nominal national refuge like the Farallones.

In the 1930s, a series of disastrous drought years coupled with the Depression focused attention on the need to protect some habitat for plunging bird populations and on opportunities to do so as land values also plunged. The federal government suddenly wanted more information about migrating birds—particularly the waterfowl that hunters value—and some kind of context for their overall management and protection. As it happened, little Lake Merritt—less posh by then after the mansion owners moved on but still offering winter refuge to waterfowl—played a big part in the enterprise.

An ornithologist named Frederick C. Lincoln had been running the U.S. Biological Survey since 1920. (The survey, a Department of Agriculture agency until its transfer to the Department of Interior in 1939, became the U.S. Fish and Wildlife Service, which runs today’s refuges.) While accumulating banding and recovery records from across the continent, Lincoln had noted the faithfulness with which ducks and geese returned to the places where they’d been banded. Among pintails and widgeons banded at Lake Merritt from 1926 into the mid-1930s, he found that nearly 97 percent of band recoveries were made west of the Rockies. In the winter of 1933–1934, more than half the ducks trapped on the lake bore bands attached there in previous years.

There was a good evolutionary reason for this. Before its conversion into urban scenery, the tidal inlet that is now Lake Merritt had been ringed by extensive seasonal wetlands. The ducks banded there must have had ancestors that frequented the area going back many thousands of years. It is encouraging, in a way, that their ancestral wetland’s conversion first to a sewer and then to a still-polluted ornamental lake didn’t deter them from returning, although, given the overall situation, where else could they go?

Lincoln coordinated this data with material from other sites showing that different migrants, like shorebirds and songbirds, also often returned to the same nesting and wintering locations. He used the information to formulate the concept of the “flyway,” a kind of avian nation “in which related migration routes . . . the lanes of individual travel from a particular breeding ground to the winter quarters of the birds that use them . . . are associated and blended in a definite geographical region.”

Lincoln designated four North American flyways—the Atlantic, Mississippi, Central, and Pacific. They were shaped like tornadoes, wide in the boreal and arctic areas where many migratory waterbirds nest, then narrowing into migration routes through the United States, Mexico, and areas south. (Most North American migrants winter in Central America and the Caribbean, although some reach South America and a few, like Swainson’s hawks, fly all the way to Patagonia.) Lincoln acknowledged that there was a lot of crossing from flyway to flyway, but aside from their soft edges, he considered them real biological entities. After World War II the new Fish and Wildlife Service came to regard them as such, making them the basic administrative units for hunting regulations and refuge management.

Lincoln’s work definitely contributed to migratory bird management. Since his death in 1960, however, ornithologists have largely discarded flyways as biological entities. One biologist who studies songbirds gave them a scathing review, calling them “misleading . . . hopeless, bogus . . . like saying the earth is flat . . . a disservice to science.”

“Small birds are broad front migrants,” he said. “They inherit an ability to fly a certain distance in a certain direction, and they don’t care if there’s an ocean or a mountain range in the way. Where’s the flyway in that?” He grudgingly granted flyways limited usefulness for counting and managing geese, swans, and cranes but doubted their value for ducks, which he said fly in inherited directions and may change breeding grounds when following new yearly mating partners.

Most ornithologists agreed in milder terms, acknowledging flyways’ significance as an administrative tool for managing species that follow strongly established migration routes. In his authoritative Ducks, Geese, and Swans of North America, Frank C. Bellrose called them “useful geographic terms . . . and political units” and cited some cases in which waterfowl migration “fit neatly into a flyway.” But: “Flyways fail to define the passage of waterfowl because they cover too extensive an area and do not delineate movements of waterfowl that are lateral to a north-south direction.”

Some avian groups, including most of the Bay Area’s seabirds, definitely transgress the flyway concept. Brown pelicans nest on islands off southern California, then wander up and down the coast looking for food. White pelicans nest on inland lakes, then move to the coast for food. Some gull and tern species also go east to lake breeding grounds after wintering here. Other gulls and terns nest here, while some winter here and then fly north to breeding grounds. Common murres and pigeon guillemots that nest here may fly north for the winter as well as south. The notoriously endangered marbled murrelet spends winters feeding at sea and summers nesting in redwoods or Douglas firs.

Even with conventional migrants, flyways are more about administration than conservation: it’s the ground and water where birds breed and feed that need to be protected, not the air they fly through. Lincoln wouldn’t find much banding information on pintails and widgeons at Lake Merritt these days. In his time, and until the early 1960s, several thousand of these dabbling ducks visited the lake in the fall. A few dozen visit it now, because there are a lot fewer ducks. Most migratory bird populations have declined steadily as habitat has disappeared.

Yet the federal government did little for Bay Area migratory bird habitat until the environmentalist uprising of Earth Day, 1970. The first federal refuge on the Bay came in 1972—Don Edwards San Francisco Bay National Wildlife Refuge, named for the congressman who helped push it past the growth boomers and through the legislature. San Pablo Bay NWR followed in 1974, through a similar process. (The Fish and Wildlife Service finally replaced the Coast Guard at Farallon NWR that year.) In 1980 the Bay Area got the only federal refuge established to protect endangered plants, Antioch Dunes NWR. A refuge established in 1992, 339-acre Marin Islands, protects the largest heron and egret rookeries in the Bay Area, although most of its acreage is under water.

Together, the four refuges on the Bay itself add up to somewhat more than fifty thousand acres of wetlands and other habitat. This protection represents massive efforts on the part of the local public. When Congress established San Pablo Bay NWR, for example, it comprised 175 acres, and local government had approved a 1,585-acre commercial and residential development nearby. One of the reasons for establishing the refuge was that 80 percent of the canvasback ducks in the Pacific Flyway were known to feed and rest on San Pablo Bay wetlands. Thousands of houses, apartments, and office buildings obviously would have impacted that. Citizens took government to court to force an environmental review and—with the help of then congressional representative Barbara Boxer and the U.S. Land and Water Conservation fund—finally saved the 1,585 acres by buying the land from a Japanese corporation for $7 million.

Yet the refuges don’t represent a massive area considering that hundreds of thousands of wetland acres disappear from the continental United States every year. Of course, state, regional, and local parks also protect Bay Area wetlands, and conservationists led by David Brower, Edgar Wayburn, and Congressman Phillip Burton saved much migratory bird habitat when they pushed the United States to establish Point Reyes National Seashore and Golden Gate National Recreation Area. The restored marshes and riparian woodlands there are a revelation. But, considering the Bay’s international significance as a unique ecological system (UNESCO designated it a biosphere reserve in 1988), one might think that the world’s richest nation could do more.

Whatever the biological significance of Lincoln’s avian nations, it’s hard to get much sense of the Bay as more than a “stop on the flyway” from the National Fish and Wildlife Service refuges here. In the Central Valley refuges, the surrounding farmland’s open horizon gives a certain feeling of integrity, albeit a truncated one. The Bay Area’s refuges can seem like museums because the surrounding artificial environment is so pervasive.

When I visited San Pablo Bay one late spring day in 2014, the lone hiking trail to the marshes at Lower Tubbs Island led past huge hay fields and spoil banks for most of its length. At Tubbs Island, the trail became impassible because a stretch had collapsed into the marsh and the NFWS lacked the money to fix it. This being the off-season, waterbirds were scarce anyway, aside from a few mallards and egrets. As I walked out to where the trail had collapsed, a buzzing roar like a swarm of giant insects erupted from the north. Looking that way, I saw dark objects zipping up a hillside like giant bees scurrying around a hive. It was like a 1950s atomic mutant movie. Then I remembered— it was the Sears Point Raceway.

The South Bay around Don Edwards Refuge is more urbanized than San Pablo Bay, but its marshes seem less constricted because more public facilities exist. When I walked the trails around the visitor center a few days after my San Pablo hike, the warehouses, hotels, and technology parks to the east seemed to fade on the horizon, although they’d looked real enough as I drove in (albeit with surprising numbers of empty parking lots and FOR LEASE signs). I saw more birds than at San Pablo—shoveler ducks as well as mallards, gulls dipping into salt ponds, swallows nesting on an old hunting shack—but the only spectacular sight was a flock of white pelicans, and they weren’t flying over the refuge but over Coyote Hills Regional Park to the north.

A wildlife kiosk on one of the restored Don Edwards marshes referred tersely to the flyway concept: “On the west coast, most birds migrate between their summer and winter homes along a migration corridor known as the Pacific Flyway.” A kiosk on the impassible trailhead at the San Pablo Refuge reflected the concept’s present status more appositely. One panel had a block of biological boilerplate about the flyway superimposed on a color blowup of a black-necked stilt (the only one I saw that day). The type was illegibly sun-bleached—a fading screed of ornithological anachronism.

There would have been more birds if marshes and riparian woodlands, instead of hay fields and buildings, had stretched the horizons. When they did, the Bay must have been more than a way station for many shorebirds and waterfowl. There was a lot more water to produce food for them, and thus more nesting. The restored wetlands at Point Reyes support summering coots, grebes, rails, green herons, blue herons, bitterns, and wood ducks, as well as egrets and mallards.

If Frederick C. Lincoln had conceived his flyways in the sixteenth century instead of the twentieth, they might have been less elegantly tornado shaped.

Still, birds will keep coming if we let them. They are resourceful creatures, like the curlew flocks that forage many miles inland at Round Valley Regional Preserve near Brentwood each fall. In Mount Diablo’s rain shadow, the oak-dotted grassland there hardly seems the place for long-billed shorebirds, but curlews eat grasshoppers as well as marsh worms and snails. Or like a pair of mallards I saw at a tiny cattle pond for a few years.

The pond dried up in summer, so they went elsewhere then, but they were back in the winter and spring. During a wet year, they raised a brood. One day, after seeing the drake on the pond, I found the female leading a troop of ducklings in a creek on the other side of the ridge above it. The next spring was a dry one, and the pond hardly held water, but the mallards still came back. I saw them one day, huddled in a cattail patch as though wondering what to do next.

—2014