Pete Marra remembers birdwatching in the woods behind his childhood home in Norwalk, Connecticut, in the 1970s, gazing up at common nighthawks as they extended their long, pointed wings and soared through the air. “They were these aerial acrobats,” he said. “They did ballet.”

By the time he got to high school, the woods had been cut down to make room for houses, and the nighthawks had begun to disappear. Today the bird has all but vanished from his old neighborhood.

“They're rare in Connecticut now. They're rare in many places,” said Marra, now an ecologist who is the director of the Georgetown Environment Initiative. “It's an empty feeling in your stomach that these same birds that you grew up with just aren't there anymore.”

Scientists like Marra have long known that birds were in trouble, having watched their favorite species fade from view. But he said they didn’t understand the scale of the crisis — until now.

For a study published Sep. 19 in the journal Science, Marra joined with other scientists and conservationists to analyze nearly five decades of population data on 529 species of North American birds. The results were staggering: Since 1970, the continental U.S. and Canada have lost more than 1 in 4 birds. The total bird population in the two countries has fallen by almost 3 billion, with grassland birds such as western meadowlarks and American sparrows and shorebirds such as green herons taking the biggest hits.

Pete Marra is an ecologist and the director of Georgetown University’s environment initiative in Washington, D.C. Courtesy of the Smithsonian Institution

The population of birds at the start of breeding season in the U.S. and Canada has fallen from just over 10 billion to a little more than 7 billion in the last 50 years, the research showed.

“We can all talk through the stories about there being fewer and fewer birds, but it’s not until you really put the numbers on it that you can really grasp the magnitude of these results,” Marra said. "We're now seeing common species that have declined, things like red-winged blackbirds and grackles and meadowlarks — species that I grew up with, that were very common when I was a kid. That is the most surprising and most disturbing part.”

‘Nature is unraveling’

The findings track with other research showing a steep drop in the numbers of insects and amphibians, and point to a broader ecological decline.

“We're making the wrong moves now to sustain nature for the future, and this is an indication that nature is unraveling and that ecosystems are highly stressed,” said Mike Parr, president of the American Bird Conservancy and a co-author of the study. “Our generation is going survive it, and probably the next generation will, but who knows where the tipping point is.”

For the new study, the researchers compiled 48 years of data gathered by volunteers who took part in large-scale bird surveys such as the North American Breeding Bird Survey, the National Audubon Society Christmas Bird Count and the International Shorebird Survey. They combined decades of survey data showing the downward trend in population with recent estimates of bird population size to determine the total number of birds lost.

The researchers also reviewed 10 years of data from the National Weather Service Next Generation Weather Radar (Nexrad), a network of radars able to detect insects and birds as they track precipitation. The radar data corroborated the survey data.

“This is an impressive paper assembling several big datasets,” Maria Dornelas, a biologist at the University of St. Andrews in Scotland, who was not affiliated with the study, said in an email. “I think the result that bird abundance in the U.S. has declined on average in the past 40 years or so is really important and needs to inform bird conservation in the U.S.”

Explaining the decline

The researchers said humans are driving the decline through the clearing of land and widespread pesticide use and by allowing domestic cats to roam outdoors.

Habitat loss seems to be the biggest issue. By clearing forests and grasslands to erect buildings, roads and farms, humans have encroached on the ecosystems in which birds thrive. And the use of neonicotinoid insecticides has fueled the decline both by poisoning birds and by eradicating insects, depriving birds of a key food. Cats are estimated to kill more than 1 billion birds in the U.S. each year.

“I think of it as death by a thousand cuts” Marra said. “If we fix the habitat problem, we would have a rebound, but there's multiple interacting threats out there that are now driving these declines.”

While climate change played only a small role in the loss of birds, the researchers said it would likely become a bigger threat in the years ahead, as rising seas inundate coastal habitats and more frequent and severe wildfires lay waste to forests.

Bright spots

While the findings are bleak, not all bird species are on the decline, said Ken Rosenberg, an applied conservation scientist at the Cornell Lab of Ornithology in Ithaca, New York, and the study's lead author.

Waterfowl have grown in number over the last 50 years, thanks to policies such as the 1972 Clean Water Act that have helped conserve wetlands.

“You have these huge flocks of ducks and geese. That didn't exist in the '60s and '70s,” Rosenberg said. “Bird populations have been shown to be resilient, and they can bounce back pretty quickly — maybe not in all cases, but at least at this point we're hopeful.”

The study authors agreed that lawmakers can help shore up bird populations by enacting legislation to conserve federal lands and curb the use of neonicotinoid insecticides. Bird lovers, they said, can help by keeping cats indoors, eating organic food to help reduce the use of pesticides and taking part in bird surveys like the Audubon Christmas Bird Count.

Future research will draw on the work of so-called citizen scientists to track the migratory patterns of bird species. Scientists want to understand the specific threats birds face as they fly across the continent in order to make recommendations for conservation efforts.

“My gut says that if we don't do something, if we don't act right now, we're going to lose more and more birds,” Marra said. “We need to think about birds as if they are Monets and Rembrandts and Homers flying around out there, because if we lose them, it's like burning down one of our greatest museums. These are things we'll never be able to see again.”

Want more stories about the environment?

Sign up for the MACH newsletter and follow NBC News MACH on Twitter and Facebook and Instagram.