(CNN) Bird populations in the United States and Canada have dropped by 29% since 1970, signifying 2.9 billion birds lost in almost 50 years, according to a new study.

The scientists involved in the study warn that like a canary in a coalmine, birds reveal environmental health. This steep loss of bird populations, including some of the most common birds like sparrows and finches, shows that human impacts on the continent's environment mean it can no longer support the wildlife systems it once did.

Normally, it's hard to track animal populations this way. But birds are much easier to monitor. The study combines almost 50 years of data, including information collected by citizen scientists and weather radar data of migratory birds from 143 stations across North America. Observations by people were shared with the North American Breeding Bird Survey at the US Geological Survey, the Canadian Wildlife Service, the Audubon Christmas Bird Count and Manomet's International Shorebird Survey.

"Citizen-science participants contributed critical scientific data to show the international scale of losses of birds," said John Sauer, study co-author at the US Geological Survey. "Our results also provide insights into actions we can take to reverse the declines."

"Multiple, independent lines of evidence show a massive reduction in the abundance of birds," said Ken Rosenberg, the study's lead author and a senior scientist at the Cornell Lab of Ornithology and American Bird Conservancy. "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."

The Baltimore Oriole.

Almost 90% of the birds lost came from 12 common songbird families like sparrow, blackbirds, warblers, finches and swallows. Shorebirds have also lost one-third of their population, and grassland birds lost more than 720 million, resulting in a 53% population reduction. Forests across North America lost more than 1 billion birds.

The extreme loss matches a similar picture emerging around the globe. The loss of grassland birds in North America is similar to a decline in farmland birds across Europe, according to the study. The study did not analyze causes of the declines. But widespread population decline is usually due to a loss of habitat, and factors that interfere with breeding and increase mortality.

Previous research has warned that domestic cats, glass buildings, pesticide use and climate change all play a part.

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

But the study also showed that some bird populations have bounced back after suffering previous declines. For example, duck, geese and swan populations have improved over the last 50 years due to conservation efforts and funding to protect wetlands. The banning of DDT has allowed raptors, like the Bald Eagle, to also improve.

A sanderling shorebird.

But these gains don't make up for the current losses because the numbers can't be offset.

Species that had been introduced in certain areas didn't fare well, either. When native and introduced birds are dying out, nothing is replacing them because the birds just aren't thriving in environments so altered by humans, the researchers said.

"It's a wake-up call that we've lost more than a quarter of our birds in the US and Canada," said Adam Smith, study co-author at Environment and Climate Change Canada. "But the crisis reaches far beyond our individual borders. Many of the birds that breed in Canadian backyards migrate through or spend the winter in the US and places farther south -- from Mexico and the Caribbean to Central and South America. What our birds need now is an historic, hemispheric effort that unites people and organizations with one common goal: bringing our birds back."

The researchers also said their estimates include only breeding populations, so the loss could be greater.

The loss of common birds has happened before, like the extinction of the passenger pigeon, according to the study. Once the most common bird on Earth, it rapidly went extinct in 1914 due to loss of habitat and excessive hunting -- and because no one was monitoring the population.

Birds are part of a well-functioning ecosystem. They're part of the food chain, they eat insects that are considered pests to humans, and they help with dispersing seed and pollination, the researchers said.

A meadowlark.

And because birds are monitored so well, the researchers fear that this may "represent the tip of the iceberg, indicating similar or greater losses" in other animal groups, according to the study.

So what can be done? The improvement in certain bird species has shown that conservation and legislation work, the researchers said. But there are other things people can do to help. Suggestions on a website launched in tandem with the study, 3billionbirds.org , include installing screens on windows to prevent birds from crashing into buildings, keeping their cats indoors, adding native plants to their yards, avoiding the use of pesticides and single-use plastics, and drinking coffee that supports bird habitats rather than destroying them.

They can even share observations of birds in their own backyard with different projects to track where birds are thriving or need help.

"The story is not over," said Michael Parr, study co-author and president of the American Bird Conservancy. "There are so many ways to help save birds. Some require policy decisions such as strengthening the Migratory Bird Treaty Act. We can also work to ban harmful pesticides and properly fund effective bird conservation programs. Each of us can make a difference with everyday actions that together can save the lives of millions of birds."