WASHINGTON (AP) — 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% to about 7.2 billion birds, according to a study in Thursday’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 modeling 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.

“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,” Cracraft said.