World / Science & Health

North American bird population has fallen by a third in 50 years


The number of birds in the United States and Canada has fallen by an astonishing 29 percent — almost 3 billion — since 1970, scientists have reported, saying their findings signal a widespread ecological crisis.

Grassland birds are the most affected because of the disappearance of meadows and prairies and the extension of farmland, as well as by the growing use of pesticides that kill insects, affecting the entire food chain.

But forest birds and species that occur in a wider variety of habitats — known as habitat generalists — are also part of the downward trend.

“We see the same thing happening the world over. The intensification of agriculture and land-use changes are placing pressure on these bird populations,” said Ken Rosenberg, an ornithologist at Cornell University and principal co-author of the paper in the journal Science. “Now we see fields of corn and other crops right up to the horizon — everything is sanitized and mechanized; there’s no room left for birds, fauna and nature.”

More than 90 percent of the losses are from 12 species, including sparrows, warblers, blackbirds and finches.

The figures are similar to declines observed elsewhere, notably France, where the National Observatory of Biodiversity estimates a 30 percent decline in grasslands birds between 1989 and 2017.

The U.S. study combined two data sources.

The first was annual surveys carried out each spring, during the breeding season, conducted by thousands of volunteers according to an identical methodology since 1970. The volunteers stop every half a mile along a 25-mile (40-kilometer) journey through the countryside, counting all the birds they see in three minutes. Researchers then compile and analyze the results.

The second source came from observations from 143 radar stations that can detect flocks of birds during their migrations that took place at night.

More than half of the North American bird species are migratory, flying either to the southern U.S. or on to Central and South America.

The radar information was less precise but also showed a decline of 13.6 percent between 2007 and 2017, with a margin of error of 9 points.

Ducks and geese are the big exception: after a period of decline, their numbers have grown since 1970.

That is thanks to increased awareness among hunters who have supported protection measures, Rosenberg said.

Ornithologists also mention other factors, such as cats left outside, and even birds dying after smashing into the windows of houses. The number killed in this way is far from insignificant. Researchers in 2014 estimated it at between 365 million and 1 billion per year.

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