More than one in four birds have been lost in various groups and habitats, in what the researchers describe as a "wake-up call."
The United States and Canada have lost more than one in four birds, a total of three billion, since 1970, culminating in what scientists who published a new study call a "widespread ecological crisis."
The researchers observed a 29% decline in bird populations in various groups and habitats, from songbirds like larks to long-distance migratory birds like swallows and backyard birds like sparrows.
"Multiple independent lines of evidence show a massive reduction in bird abundance," said Ken Rosenberg, lead author of the study and lead scientist at the Cornell Laboratory of Ornithology and the American Bird Conservancy.
Co-author Adam Smith of Environment and Climate Change Canada called the findings a "wake-up call."
Population losses are consistent with what scientists have counted for insects and amphibians.
The study, published in the journal Science, did not analyze the reason for the fall. But around the world, birds are believed to be dying more and having less success reproducing largely because their habitats are being damaged and destroyed by agriculture and urbanization.
The researchers calculated the declines using 10 years of information on migratory birds from weather radar stations and 50 years of data from the ground. Sources include citizen science from the United States Geological Survey, the Canadian Wildlife Service, the Audubon Christmas Bird Count, and the Manomet International Shorebird Study.
Grassland birds were particularly affected, with a 53% reduction in population. Shorebirds were already in low numbers and have now lost more than a third of their population. Night sky radar found that the volume of the spring migration fell 14% in the last decade.
Domestic cats, collisions with glass and buildings, and the decline in insects eaten by birds, probably due to widespread use of pesticides, also contribute to the decline in bird numbers. And climate change exacerbates those problems by altering the habitat of birds.
Not all bird species decreased. Raptors and waterfowl showed gains, likely due to focused conservation efforts, even under the Endangered Species Act.
Co-author Michael Parr, president of the American Bird Conservancy, said saving birds will require policy changes, harmful pesticide bans and funding for bird conservation.
"Each of us can make a difference with everyday actions that together can save the lives of millions of birds, actions such as making windows safer for birds, keeping cats indoors, and protecting habitat," he explained. Parr.