A report in the journal Science says there are 3 billion fewer birds in the United States, Canada and Mexico than 1970 — a 29% drop.
Conservationists call it a widespread ecological crisis.
“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 is too late,” lead author of the study Kenneth Rosenberg of Cornell University says.
More than 90% of the losses were among 12 species with the common house sparrow at the top of the list.
The experts blame the disappearance of natural meadows and grasslands in favor of farmland for the drop.
They also say pesticides are killing the insects that many birds use for food.
“We see fields of corn and other crops right up to the horizon. Everything is sanitized and mechanized. There’s no room left for birds, fauna, and nature,” Rosenberg said.
The study also cites free-roaming domestic cats and birds slamming into windows that reflect the sky.
But the study says the duck and goose population has actually grown since 1970 because of less hunting and more protective measures.
Ornithologists say the drop in bird populations can be reversed by simple measures including keeping pet cats inside, window treatments that can prevent birds flying into them, and avoiding pesticides and insecticides.