North America's skies are lonelier and quieter as nearly 3 billion fewer wild birds soar in the air than in 1970, a comprehensive study shows.

The new study focuses on the drop in sheer numbers of birds, not extinctions. The bird population in the United States and Canada was probably around 10.1 billion nearly half a century ago and has fallen 29 per cent to about 7.2 billion birds, according to a study in yesterday's journal Science.

"People need to pay attention to the birds around them because they are slowly disappearing," said study lead author Kenneth Rosenberg, a Cornell University conservation scientist. "One of the scary things about the results is that it is happening right under our eyes. We might not even notice it until it's too late." Rosenberg and colleagues projected population data using weather radar, 13 different bird surveys going back to 1970 and computer modelling to come up with trends for 529 species of North American birds. That's not all species, but more than three-quarters of them and most of the missed species are quite rare, Rosenberg said.

Using weather radar data, which captures flocks of migrating birds, is a new method, he said.

Advertisement

"This is a landmark paper. It's put numbers to everyone's fears about what's going on," said Joel Cracraft, curator-in-charge for ornithology of the American Museum of Natural History, who wasn't part of the study. "It's even more stark than what many of us might have guessed."

Every year University of Connecticut's Margaret Rubega, the state ornithologist, gets calls from people noticing fewer birds. And this study, which she wasn't part of, highlights an important problem, she said.

"If you came out of your house one morning and noticed that a third of all the houses in your neighbourhood were empty, you'd rightly conclude that something threatening was going on," Rubega said in an email. "Three billion of our neighbours, the ones who eat the bugs that destroy our food plants and carry diseases like equine encephalitis, are gone. I think we all ought to think that's threatening."

The research only covered wild birds, not domesticated ones such as chickens.

Rosenberg's study didn't go into what's making wild birds dwindle away, but he pointed to past studies that blame habitat loss, cats and windows.

"Every field you lose, you lose the birds from that field," he said. "We know that so many things are killing birds in large numbers, like cats and windows."

Experts say habitat loss was the No. 1 reason for bird loss. A 2015 study said cats kill 2.6 billion birds each year in the United States and Canada, while window collisions kill another 624 million and cars another 214 million.

That's why people can do their part by keeping cats indoors, treating their home windows to reduce the likelihood that birds will crash into them, stopping pesticide and insecticide use at home and buying coffee grown on farms with forest-like habitat, said Sara Hallager, bird curator at the Smithsonian Institution.

Advertisement

- AP