Migratory birds fly south over the U.S.-Mexico border
Migratory birds fly south over the U.S.-Mexico border
Photo: Getty

New research published in the Proceedings of the National Academy of Sciences on Tuesday shows that the climate crisis may be seriously messing with North American birds’ migration patterns, and the consequences could be dire.

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The researchers examined monitoring data from the North American Breeding Bird Survey, which biologists started in the 1960s in response to changes in bird populations. It has since been populated with massive amounts of citizen scientists’ data. By plugging that data into a species distribution model, the scientists examined how the breeding patterns of 32 eastern North American bird species shifted from 1972 to 2014. They found that over that 43 year period, different kinds of bird species’ ranges of migration have changed in different ways, with particularly bad news for birds that migrate from the tropics.

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Birds in North America can be broken into three categories. There’s resident birds—such as scavenging birds of prey like black vultures, game birds like wild turkeys, and many varieties of woodpeckers and owls—which are able to find adequate reserves of food and tolerable temperatures for breeding all year round in one region and therefore don’t migrate. Then there’s temperate birds, including cardinals and many wren varieties, which migrate south for the winter within North America. A third type of bird species, known as neo-tropical, breed in Canada and the U.S. during summer and travel to the neotropical locations of Mexico, Central America, South America, and the Caribbean each winter.

“Neo-tropical birds are the ones that undertake these really long migratory journeys... so they’re the ones people are really excited to see at this time of year in spring when they’re birdwatching,” Clark Sawyer Rushing, the lead author of the new study and a professor Utah State University, told Earther. “This includes a lot of warbler species and orioles and buntings.”

The team found that as northern latitudes have warmed, resident and temperate birds’ upper ranges have shifted northward, and their southern ranges have remained roughly the same, meaning the species cover a slightly larger range than they did in the 1970s.

Neo-tropical birds, however, haven’t been as lucky. While their southernmost range boundary has shrunk slightly, their northern boundary has not expanded poleward. That suggests they’re not adapting as well to global warming, though why that may be is an area of active research Rushing and his colleagues are looking into. Whatever the reason, it’s bad news for species already under heavy pressure.

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“That’s a bit alarming because tropical migrants as a group have declined quite dramatically over the last 50 years or so,” said Rushing.

Indeed, a September paper showed that North America has lost nearly a third of its bird population since 1970. That adds up to nearly 3 billion birds lost, and most of those losses were among neo-tropical species.

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“This paper adds a little bit of context to that by looking at the role that climate change has played,” said Rushing.

Climate change is just one factor among many that’s causing bird populations to decline. Billions are killed by pesticides and cats each year, and collisions with windows kill hundreds of millions more. But the biggest factor in bird population declines is loss of habitat due to agriculture expansion and urbanization.

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“What’s unique about these migratory species that they need high quality habitats across their entire migratory journey,” said Rushing. “They need habitats to breathe in, they need habitats to stop and rest while they’re migrating, and they need habitats to live through the winter where they spent most of the year. Habitat loss at any point along that journey can cause population decline.”

But as the planet continues to warm and the weather continues to get weirder, the climate crisis could play a larger role in mass bird deaths. That, in turn, could create havoc for other parts of the natural world. Because migratory birds regulate bug populations and pollinate plants, the reverberations of this loss will be felt throughout ecosystems. And since we rely on those processes as humans, too, it’s in our best interest to protect birds by restoring habitats and drawing down carbon emissions to slow climate change as quickly as possible.

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Staff writer, Earther

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