Bird populations in the United States and Canada have declined by 29% in the past 50 years.

Of the nearly 3 billion birds lost, many were sparrows, warblers, finches and swallows.

"Can you imagine a world without birdsong?"

If you've noticed fewer birds in your backyard than you used to, you're not mistaken.

North America has lost nearly 3 billion birds since 1970, a study said Thursday, which also found significant population declines among hundreds of bird species, including those once considered plentiful.

Overall, bird populations in the United States and Canada have declined by 29% in the past 50 years, according to the study, which authors say is a sign of a widespread ecological crisis.

"Multiple, independent lines of evidence show a massive reduction in the abundance of birds," said study lead author Ken Rosenberg, a senior scientist at the Cornell Lab of Ornithology and American Bird Conservancy, in a statement. "We expected to see continuing declines of threatened species. But for the first time, the results also showed pervasive losses among common birds across all habitats, including backyard birds."

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The findings showed that of the nearly 3 billion birds lost, most belonged to 12 bird families, including sparrows, warblers, finches and swallows. Overall, the drop was from about 10 billion birds in 1970 to about 7 billion now.

The cause is primarily habitat loss, as birds are losing the places they need to live, find food, rest and raise their young.

Other threats to birds include deaths because of free-roaming cats, collisions with glass, toxic pesticides and insect decline. Climate change compounds all of these problems and also accelerates the loss of the habitats that birds need, experts say.

"These data are consistent with what we're seeing elsewhere with other (groups of animals) showing massive declines, including insects and amphibians," said study co-author Peter Marra, the director of the Georgetown Environment Initiative at Georgetown University.

Evidence for the declines came from the detection of migratory birds in the air from weather radar stations across the continent in a period spanning over 10 years, in addition to nearly 50 years of data collected through numerous ground observations.

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However, not all species are on the decline. Some bird species, including raptors and waterfowl, showed population gains – likely because of focused conservation efforts and Endangered Species legislation, the study found.

Still, Marra said "it's imperative to address immediate and ongoing threats, both because the domino effects can lead to the decay of ecosystems that humans depend on for our own health and livelihoods – and because people all over the world cherish birds in their own right. Can you imagine a world without birdsong?"

The study appeared in the peer-reviewed journal Science.