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.

https://www.nbcnews.com/mach/science/u-s-canada-have-lost-3-billion-birds-scientists-say-ncna1055961