As climate change speeds up the arrival of spring across North America, migratory birds that travel thousands of miles to northern breeding grounds across Florida and other coastal states are having trouble keeping pace.
In a study of more than 7 million observations compiled on the site eBird, scientists found the farther the birds migrate, the more likely they are to arrive behind schedule.
“That leads to this situation where we have winners and losers,” said Morgan Tingley, a University of California, Los Angeles ornithologist whose lab led the study.
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Arriving in the waning days of spring, he said, could mean finding too few berries, bugs and other critical food needed to continue their journey or feed their young. That may drive the decline in certain species.
“We have birds that are actually really sensitive to changing climate change and therefore also sensitive to earlier springs. And then we have some birds that just appear to be insensitive,” he said.
For the study, Tingley and a team of researchers harnessed the power of citizen science by tapping into eBird, the site managed by the Cornell Lab of Ornithology. Birders use the site to catalogue where and when they spot birds, creating a massive repository where more than 100 million reports are logged every year.
“You might not think at that moment that it's actually contributing to science,” Tingley said. “But when you aggregate all those up, we can actually study this pattern of birds.”
The team looked at 56 migratory birds, and some of the most exotic to pass through Florida, to see how they might adjust their migrations with the changing climate. They zeroed in on migratory birds because changes in their behavior are easier to spot.
By examining the millions of observations from eBird, and comparing them to satellite imagery that showed the planet greening — and spring arriving — the researchers were able to tease out patterns.
“One of the things that our study has really done for the first time is actually explain why some birds are winning and why some birds are losing,” Tingley said.
Depending on how far they need to migrate, some birds are able to adjust to shifts in seasons, riding what researchers call “the green wave.” But those that travel longer distances struggle, including the cinnamon-colored veery.
The veery winters in central and southern Brazil, spending much of its life hopping around on the forest floor. But every spring, the fist-sized thrush makes the long flight north across Florida — and the middle and eastern U.S. — to northern breeding grounds, flying up to 160 miles in a single night.
“It has no clue what time spring is back in the United States,” Tingley said. “If spring is coming early one year in New York , that veery has no clue.”
Understanding whether birds adjust is key to understanding which species could be more vulnerable to shifts fueled by a warming planet. It may also help explain a dramatic decline in bird populations over the last half century. In 2019, researchers found bird populations were down nearly 30% from the 1970s, amounting to a loss of about 3 billion birds.
As seasons shift, researchers say it’s also possible that birds who overstay in northern breeding grounds could wind up having more than one round of offspring, boosting numbers. Birds could also shorten their migrations and no longer make ocean crossings.
“For example, by wintering along the coast of the Gulf of Mexico, as is increasingly seen in some species, our findings indicate that this could lead to a renewed ability of some species to track advancing spring," the researchers wrote.
But with numbers already falling, it’s likely that not all birds will make the shift, said co-author Rob Guralnick, a curator at the Florida Museum of Natural History. Species could simply falter and fade away.
“I think a lot about Rachel Carson’s 'Silent Spring' and that immediately, immediately evocative metaphor. What it would mean to have a silent spring,” he said.”Think about the dawn chorus. Think about the idea of a silent morning.”
The findings, he said, should be seen as an alarm, that the planet could be changing faster than nature can keep up.
“That’s what we’re talking about here,” he said. “It's hard for me to imagine a worse fate for our planet than to have that silence.”
