The number of birds in the United States and Canada has dropped by an astonishing 29 percent, or almost three billion, since 1970, scientists said on Thursday, saying their findings signalled a widespread ecological crisis.
Grassland birds were the most affected, because of the disappearance of meadows and prairies and the extension of farmlands, as well as the growing use of pesticides that kill insects that affects the entire food chain.
“Birds are in crisis,” Peter Marra, director of the Georgetown Environment Initiative at Georgetown University and a co-author of the study published in the journal Science, was quoted by Reuters as saying.
“Birds are the quintessential indicators of environmental health, the canaries in the coal mine, and they’re telling us it’s urgent to take action to ensure our planet can continue to sustain wildlife and people.”
Forest birds and species that occur in a wider variety of habitats — known as habitat generalists — are also disappearing.
“We see the same thing happening the world over, the intensification of agriculture and land use changes are placing pressure on these bird populations,” Ken Rosenberg, an ornithologist at Cornell University and principal co-author of the paper in Science told AFP news agency.
“Now, we see fields of corn and other crops right up to the horizon, everything is sanitised and mechanised, there’s no room left for birds, fauna and nature.”
More than 90 percent of the losses are from just 12 species including sparrows, warblers, blackbirds, and finches.
The figures mirror declines seen elsewhere, notably France, where the National Observatory of Biodiversity estimates there was a 30 percent decline in grassland birds between 1989 and 2017.
The US study combined two data sources and covered 529 species.
The first was annual surveys carried out each spring, during the breeding season, and conducted by thousands of volunteers, according to an identical methodology, since 1970.
The second source came from observations from 143 radar stations to detect the flocks of birds that migrate at night.
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 nine points.
While climate change was not the major driver of the population plunge, it was likely to exacerbate existing threats to bird populations, Rosenberg said.
The researchers said the extinction in the early 20th century of the passenger pigeon, once numbering in the billions, showed that even abundant species could go extinct rapidly.
Some types of birds showed gains. Banning the pesticide DDT allowed for the resurgence of raptor populations including the bald eagle, the researchers said.
Waterfowl management policies including wetland protection and restoration also enabled ducks and geese to thrive, they added.
“These are important examples that show, when we choose to make changes and actively manage the threats birds face, we can positively impact bird populations,” Rosenberg said.
Ornithologists say other factors have also contributed to the decline in bird numbers, including cats left to roam outside, and birds dying after smashing into the windows of houses.
The number killed in these ways is far from insignificant: in 2014 researchers estimated between 365 million and one billion birds were killed each year.