An avian apocalypse has arrived in North America. Birdsong could become a rare sound.

• Nearly 3 billion birds have disappeared across North America since 1970 - a 29% decline.
• Two-thirds of the continent's bird species are at risk of extinction due to climate change.
• According to a new study, seabird populations in Alaska experienced unprecedented die-offs following a marine heatwave five years ago.
• These findings are further evidence that the Earth may be in the middle of a sixth mass extinction.
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If you live in the US or Canada, stick your head out the window and listen closely. Don't hear any birds? That's not a coincidence.

Across North America, bird populations are declining precipitously. Three billion birds have disappeared on the continent since 1970, according to a September study - a 29% decline in the total number of birds.

"Most people won't notice that most of their backyard birds have gone, slowly but surely, over the last 50-year period," Michael Parr, a co-author of that study and president of the American Bird Conservancy, previously told Business Insider.

Sea birds, too, are dying.

Between 2015 and 2016, nearly 1 million common murres died at sea and washed ashore on beaches between California and Alaska. It was the largest mass die-off of seabirds in recorded history.

According to a study published Wednesday in the journal PLOS ONE, that unprecedented die-off was caused by increased competition in the wake of a marine heat wave in the Pacific known as "the blob."

It's the latest evidence of an insidious avian apocalypse.

"We're in the midst of a crisis," Chad Wilsey, the National Audubon Society 's chief interim science officer, told Business Insider.

'A bird emergency'

As the planet warms and people convert more wild land into farmable pastures, North America's skies are emptying.

About 90% of the 3 billion birds that have disappeared belonged to 12 families, including sparrows, warblers, finches, and swallows, the September study found. The biggest factor driving their deaths is habitat loss: Much of the birds' breeding and nesting grounds are being transformed into fallow fields as agricultural development expands.

"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," Kenneth Rosenberg, the lead author of that study, said in a press release at the time.

The Audubon Society predicted in 2014 that by 2080, 314 North American bird species would be pushed out of 50% of their habitat range. A more recent Audubon report published in October suggested that Arctic and forest birds are the most at risk because their preferred habitats are the smallest to begin with.

That report, which was based on 140 million individual bird observations, also found that 389 of 604 North American bird species are at risk of extinction due to the consequences of climate change: rising temperatures, rising seas, and more extreme weather like droughts and heat waves.

State birds like the Minnesota common loon, New Jersey goldfinch, and California quail topped the Audubon's list of birds with shrinking habitats.

"It's a bird emergency," David Yarnold, CEO and president of Audubon, said in a release.

If you're curious about which birds in your particular area are threatened, the Audubon Society offers an interactive app that allows users type in their zip code and view all the vulnerable species based on various global-warming scenarios.

"It's a way to make this existential crisis more relatable," Wilsey said.

Unprecedented seabird die-offs

Seabirds face a slightly different climate threat: marine heat waves.

That's the term for precipitous spikes in sea-surface temperatures. This abnormally warm water speeds up the metabolism of large predatory fish like salmon and cod, which prey on forage fish like herring. Those are the same fish murres eat.

So after the blob hit in 2015, "those large predatory fish got hungrier - they needed more calories per day to survive," Julia Parrish, a co-author of the PLOS ONE study, told Business Insider.

Parrish said this "effectively dialed up the competition between large fish and murres." The murres didn't come out on top.

Although other seabird populations also dropped, "the common murre die-off was by far the largest any way you measure it," Parrish said.

Climate change makes marine heat waves worse because the ocean absorbs 93% of the extra heat greenhouse gases trap on Earth. Parrish said atmospheric scientists anticipate more heatwaves hitting the same area of the Pacific in the future.

'The global biodiversity crisis has come to America's backyard'

Many researchers think the world is in the middle of a sixth mass extinction - the sixth time in the history of life on Earth that global fauna has experienced a major collapse in numbers.

According to the United Nations, up to 1 million species face a risk of extinction.

North America's 3 billion missing birds are "absolutely part of the sixth mass extinction trend," Rosenberg said.

Parr said birds are an obvious way in which this "global biodiversity crisis has come to America's backyard."

Birds' disappearance impacts more than just the US' 41 million backyard birders. (According to a US Fish and Wildlife survey, the country's bird watching industry is worth more than $100 billion.) Common birds also help control insect populations and spread plant seeds.

"Birds actually are the 'canaries in the coal mine,' giving us an indication that fundamental environmental changes are happening," David O'Neill, Audubon's chief conservation officer, wrote in October. "We may not notice day to day if our average global temperature is rising, but we will notice that there are not as many American Goldfinches as there used to be."

How to save the birds

The Audubon Society has called for the US to reach net-zero greenhouse-gas emissions by 2050.

"By stabilizing carbon emissions and holding warming to 1.5 degrees Celsius above pre-industrial levels, 76% of vulnerable species will be better off, and nearly 150 species would no longer be vulnerable to extinction," the most recent Audubon report noted.

Similarly, the UN released a proposal on Monday that calls for one-third of the Earth to be designated as national parks, marine sanctuaries, and other protected areas by 2030 in order to stop a sixth mass extinction.

"It's crucial we protect places that birds need now and in the future," Yarnold said.