Life Science North America’s great bird extinction
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North America’s great bird extinction

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A red-tailed hawk on a tree scouting for prey near a Baltimore oriole, as the number of birds in North America declines by three billion. Photo: AAP
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The number of birds in the United States and Canada has fallen by 29 per cent since 1970, scientists reported on Thursday. There are 2.9 billion fewer birds taking wing now than there were 50 years ago.

The analysis, published in the journal<em> Science</em>, is the most exhaustive and ambitious attempt yet to learn what is happening to avian populations. The results have shocked researchers and conservation organizations.

In a statement on Thursday, David Yarnold, president and chief executive of the National Audubon Society, called the findings “a full-blown crisis”.

Experts have long known that some bird species have become vulnerable to extinction. But the new study, based on a broad survey of more than 500 species, reveals steep losses even among such traditionally abundant birds as robins and sparrows.

There are likely many causes, the most important of which include habitat loss and wider use of pesticides.

Silent Spring, Rachel Carson’s prophetic 1962 book about the harms caused by pesticides, takes its title from the unnatural quiet settling on a world that has lost its birds: “On the mornings that had once throbbed with the dawn chorus of robins, catbirds, doves, jays, wrens, and scores of other bird voices, there was now no sound.”

Kevin Gaston, a conservation biologist at the University of Exeter, said that the new findings signal something larger at work: “This is the loss of nature.”

Common bird species are vital to ecosystems, controlling pests, pollinating flowers, spreading seeds and regenerating forests. When these birds disappear, their former habitats often are not the same.

“Declines in your common sparrow or other little brown bird may not receive the same attention as historic losses of bald eagles or sandhill cranes, but they are going to have much more of an impact,” said Hillary Young, a conservation biologist at the University of California, Santa Barbara, who was not involved in the new research.

A team of researchers from universities, government agencies and nonprofit organizations collaborated on the new study, which combined old and new methods for counting birds.

For decades, professional ornithologists have been assisted by an army of devoted amateur bird-watchers who submit their observations to databases and help carry out surveys of bird populations each year.

In the new study, the researchers turned to those surveys to estimate the populations of 529 species between 2006 and 2015.

Those estimates include 76 per cent of all bird species in the United States and Canada, but represent almost the entire population of birds. (The species for which there weren’t enough data to make firm estimates occur only in small numbers.)

The researchers then used bird-watching records to estimate the population of each species since 1970, the earliest year for which there is solid data.

“This approach of combining population abundance estimates across all species and looking for an overall trend is really unprecedented,” said Scott Loss, a conservation biologist at Oklahoma State University who was part of the new study.

While some species grew, the researchers found, the majority declined – often by huge numbers.

Passenger pigeons were once so numerous they blotted out the sun. Now a new wave of extinctions is at hand. Photo: Audubon Society

“We were stunned by the result – it’s just staggering,” said Kenneth V Rosenberg, a conservation scientist at Cornell University and the American Bird Conservancy, and lead author of the new study.

“It’s not just these highly threatened birds that we’re afraid are going to go on the endangered species list,” he said. “It’s across the board.”

Among the worst-hit groups were warblers, with a population that dropped by 617 million. There are 440 million fewer blackbirds than there once were.

Mr Rosenberg said he was surprised by how widespread the population drop was. Even starlings – a species that became a fast-breeding pest after its introduction to the United States in 1890 – have dwindled by 83 million birds, a 49 per cent decline.

Europe is experiencing a similar loss of birds, also among common species, said Mr Gaston, of the University of Exeter. “The numbers are broadly comparable,” he said.

The new study was not designed to determine why birds are disappearing, but the results – as well as earlier research – point to some likely culprits, Mr Rosenberg said.

Grassland species have suffered the biggest declines by far, having lost 717 million birds. These birds have probably been decimated by modern agriculture and development.

“Every field that’s plowed under, and every wetland area that’s drained, you lose the birds in that area,” Mr Rosenberg said.

In addition to habitat loss, pesticides may have taken a toll. A study published last week, for example, found that pesticides called neonicotinoids make it harder for birds to put on weight needed for migration, delaying their travel.

The researchers found some positive signs. Bald eagles are thriving, for example, and falcon populations have grown by 33 per cent. Waterfowl are on the upswing.

Prowling cats are a major cause of decreasing bird populations. Photo: Tufts University

For the most part, there’s little mystery about how these happy exceptions came to be. Many recovering bird species were nearly wiped out in the last century by pesticides, hunting and other pressures. Conservation measures allowed them to bounce back.

The sheer scale of the bird decline meant that stopping it would require immense effort, said Ms Young, of the University of California, Santa Barbara. Habitats must be defended, chemicals restricted, buildings redesigned. “We’re overusing the world, so it’s affecting everything,” she said.

The Audubon Society is calling for protection of bird-rich habitats, such as the Great Lakes and the Colorado River Basin, as well as for upholding the Migratory Bird Treaty Act, which the Trump administration is trying to roll back.

The society and other bird advocacy groups also suggest things that individuals can do. They urge keeping cats inside, so they don’t kill smaller birds. Vast numbers of birds die each year after flying into windows; there are ways to make the glass more visible to them.

-The New York Times