This hurricane season is off to a slow start, but as of early August the U.S. Climate Prediction Center still called for it to be above-normal. Between 6 and 10 hurricanes could barrel across the Atlantic basin by the end of November, the center projected, with 3 to 5 of them rated major.
How the season plays out has high stakes not only for people—hurricanes are historically the deadliest and costliest natural disasters—but also for birds. Tropical cyclones, including hurricanes, have a history of shredding habitat, which is especially dangerous for rare non-migratory species that live only on a single island or archipelago. The Bahama Nuthatch, for instance, had its population whittled down by a series of storms and hasn’t been seen since Hurricane Dorian roared through its final stronghold in 2019.
For migratory species, the impacts are more complicated and varied. Radar and tracking data have shown that hurricanes can temporarily shut down migration. Some storms can swallow up thousands of birds and spit them out far inland. In other cases, birds harness tail winds along a storm’s western flank to power-boost their journeys. But sometimes the effect is more damaging: Chimney Swifts migrating south in 2005 flew into the path of Hurricane Wilma; the following year, the population in one study was half what it was before the storm.
“It really is an unfortunate confluence of phenomena,” says Bill DeLuca, a migration biologist with Audubon’s Migratory Bird Initiative. Many birds evolved to migrate during hurricane season, he notes. “But of course the increase of storm intensity and frequency could lead to population impacts down the road.”
As a warming climate rewrites old weather patterns and provides fuel for stronger storms, hurricanes pose a complex and shifting hazard for millions of birds traveling from their North American breeding grounds to wintering areas. Driven by concerns about the toll hurricanes might take on already beleaguered bird populations, and armed with new tools to investigate their impacts, scientists are intent on better understanding this evolving threat.
Among them are researchers using weather radar to peer into the eyes of storms and measure the birdlife trapped there. As a hurricane forms, fierce winds can concentrate birds in the relative calm of the eye. In a study published earlier this year, meteorologist Matthew Van Den Broeke measured “bioscatter”—bats, bugs, birds, and other organisms—caught up in the eyes of 42 hurricanes with differing intensities. Bioscatter, which consisted mostly of birds, was higher in more severe cyclones and those coinciding with peak migration.
Van Den Broeke is also investigating whether scientists may be able to read clues in these bioscatter signatures about how storms will behave as they close in on coastal communities. “Maybe you can use something about the altitude and size of this area of bioscatter to determine if a system is strengthening or weakening,” he says. But in the meantime, the findings suggest that increasingly intense storms could corral large numbers of birds, transporting them off course and draining their energy during already taxing migrations.
While radar can’t show us whether birds die after they’re trapped, Van Den Broeke points to anecdotal observations from mariners that describe droves of birds landing on ships in the eyes of hurricanes. “A lot of times those accounts indicate that the birds are really tired,” he says. “They land on the ships and they seem exhausted.” Winds may be calm in the eye, but getting caught there could force birds into extended flights—potentially deadly for migrants running on limited energy budgets over open water. (Storm-exhausted birds have at times provided a windfall for hungry young sharks in the Gulf of Mexico.)
Jeff Buler, an ecologist at the University of Delaware, is also using weather radar to study cyclones’ avian impacts, but in a different way. He’s particularly interested in how hurricanes affect stopover habitat, the critical rest areas where birds refuel during migration. Buler studies the number of birds lifting off to resume their journeys during the moments known as “peak exodus.” Measuring these mass departures helps Buler understand a habitat’s overall value to migrants, especially how much food and shelter it provides.
In Louisiana, Buler has used radar before and after hurricanes to observe how storms affect the ability of coastal bottomland forests—considered the region’s richest stopover habitat—to host migrants. After Hurricane Katrina in 2005 destroyed trees and denuded the canopy of leaves, mosses, and berries, radar showed migrants abandoning those battered areas for higher-elevation pine woods, until the bottomland forest grew new foliage more than a month later.
Although this finding demonstrates birds’ flexibility, it also points to a potential concern: Those pine forests are worse than the bottomlands at feeding and sheltering migrants. And hurricanes aren’t the only forces pushing birds into less-than-ideal habitat; Buler says that human-caused destruction of stopover sites along the Gulf of Mexico is compounding the problem. “It gives migrants fewer choices of where to stop over and puts more pressure on remaining habitats to support migrants,” he says. “At some point this will lead to increased mortality of birds or delays in their arrival to wintering grounds.”
As scientists look to radar for a broad view of how hurricanes affect birds, others are tracking individual animals to read the fine print of these interactions. For more than 10 years, Bryan Watts, director of the William and Mary Center for Conservation Biology, has affixed tracking tags to Whimbrels, blueberry-gobbling, scythe-billed shorebirds that migrate over the Atlantic during hurricane season. In a study published last year, Watts found that Whimbrels breeding in different parts of North America had distinct migratory routes and thus different hurricane exposure. Birds that nested along Hudson Bay flew over the Caribbean and encountered significant hurricane activity. Whimbrels that bred farther west in the Mackenzie Delta followed a more eastern migration route over the Atlantic Ocean, which avoided most storms but required longer, nonstop flights across open water all the way to South America.
Although Caribbean islands serve as a safety net for Whimbrels migrating through the heart of Hurricane Alley, touching down brings its own risks. Watts lost two tagged birds to hunting during the study, and the toll across the Caribbean is much greater. “Tens of thousands of birds are shot every year, and it’s storm related,” says Watts. “I think that locals have always known that storms bring birds and it’s an opportunity to hunt.” The last Eskimo Curlew known to science was shot in Barbados in 1963 during peak hurricane season. Watts says shorebird species too small to track with transmitters use the same routes as Whimbrels, and are likely meeting the same fate, a harvest he says is unsustainable given the drastic decline in shorebirds globally.
Because the Whimbrel populations he studies have different hurricane encounter rates, Watts says they provide a good study system to understand the impact of changing hurricane seasons in the future. Will the Mackenzie Delta population remain viable if more storms track up the East Coast, for example? “I do have concerns about what the implications are for this particular pathway,” he says. “There’s no escape hatch for them.” This year, Watts tagged Whimbrels with next-generation tags, which ping the birds’ location more frequently and measure altitude and flight speed. Researchers will learn about their migrations in higher resolution, and the additional sensors will help to unravel the second-by-second decisions the birds make when encountering hurricanes.
Along with these high-tech tools, the growing popularity of community science platforms makes birders’ observations another avenue for understanding a changing hurricane season. Cornell University ornithologist Andrew Farnsworth, who uses data from eBird in his research, says records generated in the buildup, passage, and wake of tropical cyclones provide a direct link between storms and bird movement, offering insights into avian behavior in extreme situations. For example, observations of Hurricane Irene in 2011 showed an influx of White-tailed Tropicbirds transported far inland across the east coast. “It’s easy to gather data when things are ‘normal’ but very rare to acquire information in extreme cases,” Farnsworth says. “eBird offers a real opportunity to engage many people observing in many places relative to rare events.”
Although Farnsworth is concerned about the impacts of a shifting hurricane season, his worries arise mostly from how major storms add to the cumulative threats birds already face, such as feral cats, collisions with infrastructure, and light pollution. With the number of community science observations growing after every hurricane, Farnsworth says we’ve only begun to draw bigger insights about these highly adapted animals. “I do feel that there’s this unique opportunity to study how birds deal with these particular events,” he says. “I would love to know more.”
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How Bird Researchers Are Tracking the Impacts of Intensifying Hurricane Seasons - National Audubon Society
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