Save the Birds — With Doppler Radar
Doppler radar helped save the Texas forests where millions of migrating birds rest each spring.
After slogging through knee-deep water, past palmetto thickets and trumpet vines dangling from the treetops, U.S. Fish and Wildlife Service biologist Mike Lange stops short. He signals toward a gnarled live oak, straight out of the magical charm of The Shire, its trunk the width of a car. Crumpled resurrection ferns line its branches, waiting to sprout in green abandon with the next rains. Nearby, the trunks of an elm and a water hickory wrap around each other like a sculpture of intertwined lovers.
Lange is rightly proud of these woods. Over the past 20 years, he has been largely responsible for orchestrating the conservation of what is known as the Columbia bottomlands, low-lying hardwood forests lining the southern portions of the Brazos, Colorado and San Bernard Rivers before they cross the Texas coast and spill into the Gulf of Mexico. This particular piece of the bottomlands — the Dance Bayou tract of the San Bernard Refuge — contains one of the South's few remaining old-growth forests.
The Columbia bottomlands are also an amazing hotspot for migratory birds, with an estimated 40 million to 80 million individual birds of 240 species using the area. Many of them are spring migrants stopping to rest and eat after crossing the Gulf of Mexico before heading farther north.
But no one understood the significance of this thicketed haven for birds until a good-natured Cajun named Sidney Gauthreaux came to visit in the early 1990s. It was just after the National Weather Service installed the first Doppler radar on the Gulf Coast south of Houston, and meteorologist Bill Read — now director of the National Hurricane Center — invited Gauthreaux to check it out. What they discovered catalyzed the conservation effort Lange and the Fish and Wildlife Service have led since then.
"The displays were never more spectacular than when we saw the Columbia bottomlands near Houston," Gauthreaux says, his passion for birds and science shining through a distinctive Cajun lilt. "We knew it had to be a very special area."
Birds and radar have a storied history, but the relationship has not always been synergistic. During World War II, England established radar stations along its coastline, providing early warning when the fighters and bombers of the Luftwaffe were crossing the English Channel. On more than one occasion, unidentified radar signals caused widespread panic among British radar operators. When these mystery echoes appeared, which was always at night, they resembled small aircraft heading toward the coast of Britain. Sometimes masses of these echoes covered the radar screen, but they always vanished by morning, and no attack ever followed. The British military started calling the false returns "angels" — that is, the souls of dead soldiers headed home. The term made its way into the Oxford English dictionary by 1947 and soon became common lingo for any unexplained radar interference.
Eventually, radar operators came to believe that atmospheric discontinuities caused angel echoes. A few advanced the idea that the angels might be birds, but most considered the idea preposterous. A secret British government document, now declassified, says, "The extreme reluctance in accepting the view that birds are responsible for angel phenomena appears to arise from the feeling that an object so small and soft as a passerine bird could not possibly produce a detectable radar echo."
The English military's top brass turned eventually to prominent British ornithologist David Lack, who began gathering year-round data on migrating birds and soon correlated bird movements with the angel echoes. But by then, the war was over, life moved on, and talk of angels passed from the popular lexicon.
The U.S. National Weather Service established an early-warning radar system in the late 1950s, and Gauthreaux, then a high-school-age birdwatcher, became intrigued by it. He swore he could occasionally detect birds on the black-and-white images of those early radar systems, known as Weather Surveillance Radars-1957, or, more casually, WSR-57. He eventually paired his interests and helped pioneer the field of radar ornithology while in graduate school.
Gauthreaux accepted a professorship at Clemson University in 1970 and proceeded to document nearly a 50 percent decline in the numbers of trans-Gulf migratory songbirds between 1967 and 1987. (No new calculations have been done since then, but anecdotal evidence suggests a continued decline.) Early black-and-white radar had severe limitations, and Gauthreaux's research didn't really take off until the 1990s, when the National Weather Service released a "next generation of radar," also called NEXRAD, WSR-88D or — in the terminology most used today — Doppler-pulse radar.
"With WSR-57, you'd get single-frame shots," he explains. "It wasn't connected to a computer or anything." Today, scientists — and birdwatchers — can stream radar images live on their computers, enabling them to see exactly where groups of birds are at any given time. That is, if they know what to look for.
Radar works by emitting electromagnetic waves that travel until they bounce off an object in their path. By measuring the time it takes for the wave to return to the source, scientists can determine an object's distance and size. The bigger the object — whether an enemy plane or a rainstorm — the more of the wave gets reflected to the source. Older radar could not detect an object's speed, but newer radar does by making use of the Doppler Effect.
The Doppler concept is illustrated by the rising pitch of an ambulance siren as it approaches a listener and the falling pitch as the ambulance recedes into the distance. The sound waves are actually shifted as the moving ambulance approaches, making them arrive more frequently at a listener's ear and seem to be more highly pitched and vice versa for the receding vehicle. This frequency-shifting phenomenon works for any type of wave, including the electromagnetic waves emitted by a radar source.
The Doppler radar collects three types of data from the waves that come back to its receiver: reflectivity, velocity and a wind profile. From these data, scientists create the radar images seen on the news describing the intensity of a weather event, its directional movement and the wind speed. Meteorologists usually filter out echoes caused by birds, insects and even dust, occasionally talking about "ground clutter." Gauthreaux flipped that around, filtering out the weather and looking at echoes caused by the birds. "They used radar to quantify the amount of rainfall; the stronger the storm, the more rainfall," Gauthreaux says. "I used that same concept to quantify birds. Migratory birds are just big drops of water."
Meteorologists use pulse-Doppler radar, which sends out an intermittent rather than continuous signal, allowing a finer-scale detection of objects. Distinguishing birds from precipitation on a radar image is not difficult. "Birds show a fine stippling pattern while weather forms huge chunky echoes," Gauthreaux says. By studying patterns on radar images, he has learned to distinguish different types of birds — hawks, songbirds and water birds for example — as well as calculate the densities of flocks. Scientists can even study insects using Doppler, and in fact, ornithologists using radar have to remove the "noise" caused by the insects (and vice versa, for the bug scientists).
Ironically, Gauthreaux had to convince meteorologists all over again that birds caused some of the unexplained radar interference. Most of the people who had worked with WSR-57 had retired, and the new echelon of radar experts wasn't familiar with birds. Gauthreaux had figured out that birds flying faster than wind were artificially inflating the wind speeds calculated from Doppler radar. "I took a lot of grief because a lot of meteorologists said things like, 'If there were that many birds out there, we'd be up to our nose in bird crap,'" he says. "They had to ultimately eat crow because we now know those algorithms are very sensitive to picking up birds."
It was not long before Gauthreaux had used the new radar to solve one of ornithology's long-standing puzzles: Where did spring migrants from Mexico and Central America stop after crossing the Gulf of Mexico on their way north? The birds always eventually showed up at nesting habitats in northern North America. And although birds practically fall out of the sky if they experience bad weather when crossing the Gulf — a phenomenon known as "fallout" — in better weather they do not always show up in great numbers in the small coastal woodlots that have become renowned birding hotspots, including Texas' High Island sanctuary and Sabine Woods.
No one really knew whether the birds stopped somewhere en masse or kept going.
Birds land piecemeal, but they take off en masse, usually just after dusk. Pulse-Doppler radar images from that time of day showed intense concentrations of birds along the forested rivers of coastal Texas. Gauthreaux and other local biologists visited the forests and found a Shangri-la for weary migratory birds. "Right quickly," Gauthreaux says, "we discovered that radar picked up birds coming out of stopover habitats. That's proved to be a very useful tool, leading to discovery of important conservation areas."
From high in the treetops comes the crystal-clear call of a Prothonotary warbler, a brilliant yellow bird that somehow remains camouflaged in the trees. Lange points out the calls of a tufted titmouse and a white-eyed vireo. A red-shouldered hawk takes flight. But the woods of the Columbia bottomlands are not just shelter for birds of prey, songbirds and water birds. They boast a great diversity of tree species — ash, maple, pecan, hackberry, sycamore, birch, cypress, water oak and dozens of others. The multitiered canopy allows insects to thrive, providing food for most of the birds. Tree flowers, berries and fruit feed other species.
Historically, this coastal floodplain contained more than 700,000 acres of hardwood forest, but 75 percent has already been lost, much of it to growth in the Houston region. Named after a colony that Stephen F. Austin settled when Texas was still under Spanish control, Columbia later became the first capital of the Republic of Texas. Some still refer to the forests as Austin's Woods.
After Gauthreaux discovered the importance of Austin's Woods to birds, he began giving talks to the Texas conservation community. "People saw this and said, 'Good gracious alive, we've got to protect those things,'" he says. "Once we publicized this, everything started to fall into place."
In 1995, the Fish and Wildlife Service began an initiative to prevent the remaining forests from succumbing to urban sprawl and in 1997 formalized its plan just in time to receive the first land donation — the 657-acre Dance Bayou tract. Despite early resistance to what some in the generally conservative state of Texas worried was a federal land grab, the project has proceeded with almost no opposition.
To conserve as much biological diversity as possible in this highly fragmented landscape, the Columbia Bottomlands Conservation Plan set out to create an integrated network of satellite tracts, rather than one larger contiguous reserve. (A similar approach is used in Texas' Big Thicket National Preserve, a couple of hours east.) Three National Wildlife Refuges preserve Columbia Bottomlands habitat — Big Boggy, Brazoria and San Bernard. New tracts in outlying areas become, for administrative purposes, part of the San Bernard refuge.
These three refuges have received donations, entered into land conservation easements and purchased land outright with cooperation and financial help from the Houston Audubon Society, The Nature Conservancy, the Gulf Coast Bird Observatory, the National Wildlife Federation and the USDA's Natural Resource Conservation Service. The Texas General Land Office has made funds available for land purchase through the Coastal Impact Assistance Program, which allocates money from offshore oil and gas development for coastal projects.
All told, the Fish and Wildlife Service has preserved 19,000 acres since 1997, with a goal of 70,000 acres. In the long run, the federal government not only intends to reforest and improve habitat on the land it has acquired; it plans to work with private landowners to do the same.
Ephemeral radar angels may have inspired the conservation of the Columbia bottomlands, but the project has brought concrete benefits to the people living around them. In addition to their recreational value, the preserved forests provide a measure of flood control by storing excess water after heavy rains. They protect against damage from hurricanes. Ancient trees store and sequester carbon that would otherwise be released into the atmosphere as a greenhouse gas.
Some of the benefits of this conservation effort are financial. The forests help clean the water flowing through them and into the Gulf of Mexico estuaries, supporting multimillion-dollar shrimp and fishing industries. The Texas Gulf Coast is also one of the "birdiest" places in the world, and wildlife watching is a very big business. In 2006 (the latest year for which figures are available), it brought $2.9 billion in revenue to Texas, up from $1.2 billion in 1996.
In the end, though, the people responsible for saving these forests admit the effort was born in emotion. There is, after all, something awe-inspiring about a tree that has a trunk thicker in diameter than a car is wide.
"Any two slices of these woods are as different as pecan and cherry pie," Lange once wrote in a Trust for Public Land newsletter. There are upland forests, oak motts, cypress swamps and coastal prairie. Canebreaks or bamboo thickets once grew so high you could not see a horse galloping through; very few remain. "To the casual observer, I must admit that it probably looks pretty unspectacular," he continued. "To me, this land is an undiscovered place filled with mystery."