Birds are the planet's superheroes, built for survival.

The ice of Antarctica doesn't faze them. Nor does the heat of the tropics. They thrive in the desert, in swamps, on the open ocean, on sheer rock faces, on treeless tundra, atop airless mountaintops, and burrowed into barren soil.

Some fly nonstop for days on end. With just the feathers on their backs, they crisscross the hemisphere, dodging hurricanes and predators along the way, arriving unerringly at a precise spot, year after year.

They have penetrated nearly every ecosystem on Earth and then tailored their own size, habits, and colors to each one, pollinating, dispersing seeds, controlling bugs, cleaning up carrion, and fertilizing plants.

But for all their superhero powers, birds are in trouble.

Globally, one in eight—more than 1,300 species—are threatened with extinction, and the status of most of those is deteriorating, according to BirdLife International. And many others are in worrying decline, from the tropics to the poles. (Read about threatened species on each continent.)

In North America's breadbasket, populations of grassland birds such as sweet-trilling meadowlarks are in free fall, along with those everywhere else on the planet. Graceful fliers like swifts and swallows that snap up insects on the wing are showing widespread declines in Europe and North America.

Eagles, vultures, and other raptors are on the wane throughout Africa. Colonies of seabirds such as murres and puffins on the North Atlantic are vanishing, and so are shorebirds, including red knots in the Western Hemisphere.

While birds sing, they also speak. Much of their decline is driven by the loss of places to live and breed—their marshes, rivers, forests, and plains—or by diminished food supply. But more and more these days, the birds are telling us about new threats to the environment and potentially to human health in the coded language of biochemistry.

Through analysis of the inner workings of birds' cells, scientists have been deciphering increasingly urgent signals from ecosystems around the world.

Like the fabled canaries that miners once thrust into coal mines to check for poisonous gases, birds provide the starkest clues in the animal kingdom about whether humans, too, may be harmed by toxic substances.

And they prophesy what might happen to us as the load of carbon-based, planet-warming gases in the atmosphere and oceans climbs ever higher.

View Images During the summer months, Atlantic puffins, also known as common puffins, live in cliffs along the North Atlantic. Colonies of seabirds such as puffins and murres are vanishing. Photograph by Cyril Ruoso, Minden Pictures/Corbis

"And No Birds Sing"

Rachel Carson was the earliest and best known scientist to link the fate of birds to that of humans. Alerted by reports of sharp declines in birds of prey and songbirds, she began to examine the effects of the pesticide DDT. It was the first modern synthetic pesticide, in wide use after World War II to control mosquitoes and other insects.

Her book Silent Spring, published in 1962—the title echoes the poet John Keats's celebrated line "And no birds sing"—explained that DDT moved up through food chains, from the insects it was designed to kill to the creatures that ate them. It accumulated inexorably in tissues, organs, and fat in top predators such as peregrine falcons, ospreys, bald eagles, and pelicans

By 1972, after public uproar, DDT was banned in the United States and eventually banned around the world except in malaria-prone countries, mostly in Africa.

Yet DDT's legacy remains. And, again, birds are telling us this tale: A recent study reported that birds of prey in South Carolina still carry as much DDT and other legacy pesticides in their bodies as they did before such chemicals were banned in the 1970s, "suggesting exposure has not declined substantially over the past 40 years."

Globally, one in eight—more than 1,300 species—are threatened with extinction, and the status of most of those is deteriorating.

And in the small town of St. Louis, Michigan, near an old chemical plant, robins are still dropping dead of DDT poisoning, registering some of the highest levels ever recorded in wild birds.

The idea that birds tell us about our own health has gained even more scientific traction in the decades since Silent Spring as biochemical analysis has become more precise. Much of that work stemmed from studies Canadian Wildlife Service toxicologist Glen Fox and others conducted on the Great Lakes, the world's first and biggest testing ground for contaminants and birds.

Fox's work began with tales from terns and other fish-eating birds. He found high levels of polychlorinated biphenyls (PCBs) in the lakes and their sediments, and enlarged thyroids that were producing little hormone in the birds. Substances that build up in food webs just like DDT, PCBs were banned in the United States in 1978, with the rest of the world to follow.

By the late 1980s, there was so much research about chemicals in the Great Lakes that zoologist Theo Colborn, then at the World Wildlife Fund, began examining the studies to see if she could discern a big picture.

The results were stunning: The Great Lakes's top 16 or 17 bird predators were vanishing. The problem stemmed from assaults on the endocrine system, which controls hormones and reproduction. And that, in turn, was linked to man-made substances in the water and prey. So, birds' ability to reproduce crashed in multiple ways. The concept of the "endocrine disruptor" was born.

"The birds really told the story, elegantly," said Colborn, who co-authored the 1996 book Our Stolen Future, which chronicled the threats of hormone disruption.

View Images A flock of red knots takes flight from the Snettisham Reserve in Norfolk, England. Shorebirds such as red knots are decreasing in the Western Hemisphere. Photograph by FLPA/Andrew Mason, Corbis

Proxies for People

Studies have suggested that those same chemicals also may be altering human hormones. A pregnant mother's load of chemicals passes to her baby while it is still in the womb, with evidence mounting that chemicals can alter development of a baby's brain and its reproductive and immune systems, leading to lower intelligence, behavioral problems, and reduced fertility.

Some studies suggest a link between endocrine disruptors and a greater risk of prostate and breast cancers and other diseases. Some research even suggests chemicals can switch genes on and off, affecting grandchildren and great-grandchildren.

When it comes to chemicals and broad planetary changes, birds have shown us that they are in a unique position to tip us off to health threats.

Birds are highly visible. People track them, notice them, care deeply about them. Of all the nonhuman creatures on Earth, birds are by far the most closely scrutinized, said Nicola Crockford, international species policy officer with BirdLife International in England. That translates into a robust body of knowledge about how and where birds live, a baseline for scientists seeking to monitor change.

Looking at birds gives humans the unsurpassed ability to identify and quantify chemical threats across time and space around the globe, noted Christy Morrissey, an ecotoxicologist at the University of Saskatchewan in Saskatoon. "Birds can tell us a lot about what's going on around us that we might not be able to see," she said.

Perched atop many food webs, birds of prey such as eagles and falcons soak up chemicals from the things they eat. That means looking at birds is a proxy for looking at plants, insects, fish, and small mammals over time. Not only that, but about one in five birds migrates, so those birds are sampling pollutants in many parts of the world.

Scientists can capture birds, test them, band them, let them go, and then catch them years later to see what's changed. Birds also normally maintain relatively stable numbers, unlike small mammals. So when their populations take a dive, it means something noteworthy is going on.

Many birds also live a long time—for eagles and owls, decades—meaning scientists can study a bird's life cycle and then extrapolate what would happen to a human exposed to the same chemicals from birth to death, Morrissey said. Reading birds is a reasonable stand-in for a human epidemiological study, especially when it comes to the endocrine system, she added.

Today, studies on how endocrine-disrupting chemicals affect birds is a main plank of future research that may also have implications for human health. (Related: "4 Videos: Threatened Birds Face Polar Bears, Poop-Sniffing Reporters")

View Images The Verreaux's eagle, also known as the black eagle, hunts for food above Kgale Hill in Botswana. Birds of prey such as eagles and falcons soak up chemicals from the things they eat. Photograph by FLPA/Andrew Mason, Corbis

Beyond DDT and PCBs

In the prairies of Canada, Morrissey is trying to build up a picture of where sanderlings, red knots, and semipalmated sandpipers are picking up contaminants as they travel. Then she's tracking those chemicals through the birds' life span, examining whether chemicals affect their ability to fatten up and therefore sustain a long migration.

She's also looking at whether the chemicals affect brain development, robbing the birds of the ability to navigate and to know when to molt. Early results from birds dosed in captivity in the first days of life say the chemicals do have those effects. In other words, she's looking at whether the chemicals affect not just the birds' ability to reproduce, but also their ability to thrive. "If they're not able to fatten, they won't make it," she said.

Morrissey and Pierre Mineau from Carleton University in Ontario, an expert on pesticide ecotoxicology and its effects on birds, are also at the forefront of research globally on the newest class of pesticide, the neonicotinoids or neonics for short. Mineau said he was originally relieved when neonics replaced organophosphates, which are ferocious bird-killers, but that his research on neonics, which includes a report for the American Bird Conservancy, has him concerned.

Looking at birds gives humans the unsurpassed ability to identify and quantify chemical threats across time and space around the globe.

Neonics are extremely persistent in the environment and are water soluble, which means they move around, he said. The compounds also take down nearly any insect or crustacean that comes along.

Other chemicals are immediately life-threatening. Rat-killing poisons can cause agonizing death not just in rodents, but also the birds that eat them. In Southeast Asia, tens of millions of vultures have perished from feasting on carcasses of livestock treated with diclofenac, an anti-inflammatory drug. Three vulture species now are teetering on the edge of extinction.

Adding to their burden, birds are contaminated with a whole new spate of pollutants, such as perfluorinated compounds, or PFCs, used to manufacture such substances as Teflon and stain-resistant coatings. Brominated chemicals used as flame retardants in furniture foam and electronics are collecting in bird tissues, just like PCBs.

Kestrels exposed in laboratories to PFCs have fewer chicks, smaller eggs, and some behavior issues, such as bad parenting skills and more aggression in males. It all adds up to a load of dozens of chemicals, many with consequences still unknown.

In Sweden, for example, ornithologists are racing to figure out why white-tailed sea eagles on the coast of the Baltic Sea, devastated by DDT and PCBs in the 1970s, are again experiencing thin shells and deformed embryos, said Cynthia de Wit, a professor of environmental science at Stockholm University who specializes in human and wildlife exposure to synthetic chemicals. "It's very alarming; we really don't know why," she said.

Scientists also are closely examining the effects of heavy metals such as mercury and lead. Lead, sometimes lethal to birds of prey that ingest it when they eat gut piles left by hunters, also seems to have subtle effects, perhaps interfering with their ability to navigate around obstacles.

View Images The last male passenger pigeon, or wild pigeon, died in 1912. About 150 bird species, including wild pigeons, have gone extinct at the hand of humanity. Photograph by Bettmann/CORBIS

Vigilant Sentinels

Humans have relied on birds' superpowers for millennia. Imagine forest-dwellers of ancient times, anxious to avoid snakes and jaguars, listening for the alarm calls of sharp-sighted, high-flying, omnipresent birds. Think of medieval sailors, following fish-eating birds to find out where they should throw their nets.

Throughout history, humans have considered birds to be our protectors, our vigilant sentinels, writes the Nobel laureate immunologist Peter Doherty in his 2012 book Their Fate Is Our Fate: How Birds Foretell Threats to Our Health and Our World. "Way back to mythological times, guard duty has been part of the avian job description. Gods with the body of a man and the head of a bird, like the ibis, falcon, hawk or heron, watched over the ancient Egyptian," he wrote.

In many Native American and other indigenous cultures, birds are messengers sent by the creator, or symbols of change, or protectors and healers. Today they play that role in a non-spiritual sense: They send warnings to tribes about the health risks of eating fish tainted with industrial pollutants.

Birds also herald the presence of pathogens, such as avian influenza and West Nile virus, noted Nicholas Komar, a biologist with the U.S. Centers for Disease Control and Prevention in Fort Collins, Colorado, who specializes in vector-borne diseases. When birds are found dead of West Nile, it's proof humans also are at risk. Infected birds don't transmit the virus to humans—mosquitoes do—but they are a sign that it is present in the environment.

But apart from data points, birds also provide us with sheer joy—from their songs and striking colors, and from the spectacle of watching them swoop through the air. "Which of us has not wished we could do that?" asked John Fitzpatrick, director of the Cornell Lab of Ornithology in Ithaca, New York. He said humans intuitively respond to birds' colors and varied voices, which signal that the year is marching on. "They move with the seasons. It's a major annual heartbeat we feel."

And yet, in the past five centuries, about 150 bird species have gone extinct at the hand of humanity, including the passenger pigeon and the dodo, according to research by Duke University biologist Stuart Pimm. That rate is speeding up and will be ten times higher by the end of this century if trends persist, his study calculates.

View Images Three birdwatchers in Florida's Everglades National Park take part in the Audubon Society's Christmas Bird Count in 2006. The ranges of many North American bird species that stay through the winter have shifted as much as 200 miles north over the past 60 years. Photograph by Joe Raedle, Getty

Omens of a Dangerous Future

The wild card for birds, the biggest risk of them all, with the potential to magnify all past and future threats, is the high-carbon world humans have created through the burning of fossil fuels for energy. Scientists are struggling to chronicle the intricate layers of fallout from climate change—and to glimpse once again what birds foretell about humanity's fate.

Frank Gill, who wrote the textbook Ornithology and was president of the National Audubon Society, said the scientific effort has shifted dramatically from the time when Rachel Carson's work on chemicals set the standard. Today, biologists are examining complex, continental effects of climate change on birds' abundance and distribution.

For instance, brown pelicans, taken off California's endangered species list in 2009, are in the throes of a catastrophic breeding failure this year, said Dan Anderson, professor emeritus of wildlife biology at University of California, Davis, and an expert on ecotoxicology and marine ornithology. The cause appears to be an El Niño event, with its warm ocean currents and high winds. El Niños are expected to intensify and become more common in our carbon-destabilized world.

View Images A lone African spoonbill wades in waters at the Ol Pejeta Conservancy, a 90,000-acre wildlife sanctuary in Kenya. Migratory birds such as spoonbills are under threat as habitat diminishes. Photograph by Richard Du Toit, Minden Pictures/Corbis

Audubon's Christmas Bird Count found that the "center of abundance" of more than half of North American bird species that stay through the winter has shifted as much as 200 miles north over the past 60 years, a response to warmer average temperatures, said Geoff LeBaron, an ornithologist with the National Audubon Society and international director of the Christmas Bird Count.

And a study of 40 western North American songbird species found that those inhabiting high elevations are moving farther up, rather than farther north, to flee the heat, said David King, a research wildlife biologist at the U.S. Forest Service's Northern Research Station in Massachusetts. Inevitably, they will run out of places to go.

The omens from the birds are not easy to read. But so far, they are telling us that this world is shifting where they can live, forcing them to change the timing of their migrations and nesting, making their food harder to find, and perhaps fostering diseases such as the deadly West Nile virus.

The wild card for birds, the biggest risk of them all, is the high-carbon world humans have created through the burning of fossil fuels for energy.

Birds have many superpowers that humans can only envy. But we have extraordinary powers, too: the ability to alter the chemistry of the air and the sea, and to create synthetic substances that live longer than we do.

We also have the power to make sure birds continue to sing. "Birds do recover," Fitzpatrick said, "if we pay attention to what they're saying."

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