Birds and hurricanes have always had an annual life-and-death struggle. Survival has never come easy for birds, be they migratory land birds, shorebirds or birds that spend most of their time over open water. But some years are especially treacherous, especially for migratory land birds on their journey from breeding grounds in North America to winter homes in the tropics.
In 2017, for example, two of the most powerful storms ever recorded impacted the birds' eastern flyway, the path that takes them through Florida, and their central flyway through Alabama, Louisiana, Mississippi and Texas. This year, Hurricane Dorian not only sat over the Bahamas as a Category 5 storm but also continues to push birds on the eastern flyway further inland.
The effects of these hurricanes on migration patterns are being watched closely by a group of researchers who several years ago launched a project called BirdCast to understand how migratory land birds use stopover habitat on the northern coast of the Gulf of Mexico. It's a way for researchers to assess where migratory land birds are stopping en route to the tropics and how storms are modifying birds' migratory movements.
Despite their destruction, powerful hurricanes like these offer a unique opportunity to study birds' movements.
"We might be able to say something about the impact of Irma as it moves through Florida," said Jeff Buler, an associate professor of wildlife ecology in the Department of Entomology and Wildlife Ecology at the University of Delaware, at the time. Updated Doppler weather radar gives them that capability because it reveals what he calls bioscatter, animals the radar detects and distinguishes from precipitation. Even with this advanced technology, though, they aren't able to determine how many birds might have been killed by the force of the winds or have been carried out to sea and drowned. That sort of information would require telemetry tags on specific populations of birds.
With the substantial information they've been able to accumulate, though, as well as data from previous hurricanes, they are able to assess the impact of a hurricane on the fall migration.
The passage of Hurricane Dorian
Winds and waves from Hurricane Dorian beached this boat in Marsh Harbor, Great Abaco, in the Bahamas. These forces of nature also move birds, their food sources and their habitats. (Photo: BRENDAN SMIALOWSKI/AFP/Getty Images)
When a storm of this size comes close to shore and stays close to shore for such a long period of time, it seriously impacts local and transient bird communities, according to BirdCast.
As with Hurricane Irma, songbirds affected by this hurricane were traveling the Eastern flyway on a route that took them through Florida and then across the Caribbean and into Central and South America.
"These birds are very generally thrushes, warblers, flycatchers and sparrows," Buler said of Hurricane Irma, but it's also true of any hurricane that follows this path. The migration route takes advantage of westerly fall winds for these species. Other groups of birds also migrate along this flyway, including raptors, waterfowl, shorebirds and wading birds, Buler said. The migration is called a loop migration because it's a route that will bring the birds back to the United States in the spring across the Gulf on the central flyway zone and into Alabama, Louisiana, Mississippi and Texas.
But the birds faced a dual threat during the height of the fall migration in September from the brunt force of hurricane-force winds, Buler said. One threat was the loss of food resources, like insects or fruiting fall flowers that have been stripped of vegetation. The other was the possibility of birds being carried off course by the storm, perhaps even back to the starting point of their migration!
Birds can be carried off course through a phenomenon Buler calls "entrainment" in the eye of the hurricane. That happens when seabirds such as sooty terns, gannets, frigatebirds and petrels get trapped in the eye of the hurricane while it's over water. While a hurricane is at sea, ocean-dwelling birds seek shelter in the eye and just keep flying inside the eye until the storm passes over the coast where they'll take refuge on land. This phenomenon is why birders flock to areas struck by hurricanes. The storms afford them the opportunity to spot species of birds in places where they're not supposed to be.
"We still do not fully understand many of the mechanisms involved in birds ‘entrainment’ and eventual deposition by storms, which is a primary reason for our interest in observations of species associated with these storms," explained the BirdCast site.
What we learned from Hurricane Irma
Hurricane Irma's winds are so strong that birds face the possibility of being carried off-course by the storm, perhaps even back to the starting point of their migration. (Photo: Chip Somodevilla/Getty Images)
Another impact of Irma that Buler and his fellow researcher Wylie Barrow, a wildlife biologist with the U. S. Geological Survey in the Wetland and Aquatic Research Center in Lafayette, Louisiana, monitored is which birds get trapped in the bands of the storm and where the winds take
them. "Those bands are like a riptide that carries you away," Buler says. Just
as a swimmer can't fight the current of the riptide, birds that get caught
in the bands can't easily get out of them. As a result, they can be carried 100 miles or more off their intended course.
"This happened in Super Storm Sandy," says Buler. "We have evidence that some land birds that were migrating through Florida during Sandy may have gotten swept up and then deposited back up in Newfoundland and Maine." Cornell Lab's BirdCast project intensively covered Super Storm Sandy's impact on birds and collaborated with Buler on analyzing some of the data on bird movements resulting from the hurricane. Here's a report on some of the findings.
BirdCast also tracks the hurricane's impact on migratory birds, seabirds and shorebirds. "I think that understanding the ways that animals respond to extreme situations is a valuable area of research, especially given the current path of humanity in terms of our rapidly changing climate," says Andrew Farnsworth, a research associate at the Cornell Lab of Ornithology. "Hurricanes, while devastating from an economic and humanitarian perspective, do provide a unique chance for us to monitor how birds in particular respond to such extremes. We are still in the infancy of understanding both the mechanisms and the means by which such storms and transport of birds by them operate, but every storm that passes provides the opportunity to learn a bit more."
For the migratory land birds on the eastern flyway that survive Irma's winds and rains in Florida and continue their migration to the Caribbean and beyond, their problems are far from over. Numerous islands in the northern Caribbean were reduced to rubble when the hurricane, a Category 5 at the time, barreled over them. "Several migrants will use the Caribbean islands as a stepping stone stopover on their way to northern South America," says Barrow. But he adds, "Many other land bird migrants stop and winter in the Caribbean islands. They are going to be hit with reduced food resources during their fall migration in Florida and then again when they get to their wintering grounds."
Why Hurricane Harvey was different
As with other storms, Hurricane Harvey affected migratory land birds in two ways. The force of Harvey's winds stripped foliage and food resources — fruit and insects — from trees. But because Harvey was a slow-moving storm and doubled back over storm-ravaged areas, it produced extensive flooding that covered leaf litter used by foraging birds.
"We know from our previous studies that most migrants, about 55 percent of the 70 or so migrant songbird species that we studied, a little over half of their primary foraging substrate is live foliage," Barrow said. "So, with the wind stripping away the foliage, epiphytes and vine tangles where they are searching for invertebrate food, there is going to be less food.
"But for about 20 percent of these migrants, their primary foraging location is in the leaf litter on the forest floor," he added. "If you think of the broad landscape that was covered with water from Harvey — which some are saying was as big as one of the Great Lakes — you've lost a lot of foraging substrate for those species of migrants that require leaf litter."
Some of the ground foragers and those that rely on vegetation in the lower understory thickets affected by the flooding include the ovenbird, Swainson's warbler, Kentucky warbler and some of the thrushes. (The Kentucky warbler is on the State of North America's Birds 2016 watch list, and it and Swainson's warbler are on the National Audubon Society's 2007 watch list.)
These migrants are very adaptive, Barrow said, pointing out that on their long-distance migration, they encounter different habitats all the time. "In fact, adds Farnsworth, "the very reason migration exists is because birds are adapting to changing environments and atmosphere over many time scales, including the evolutionary time scale."
"Most species are pretty flexible in their foraging strategies and in their abilities to forage and find food in different locations because they do that all of the time during these movements," Barrow said. "Typically, if a migrant is in a stopover site that doesn't have adequate resources, it will move to a stopover site that has better resources. This will be hard at the western part of the Gulf for them to do."
"I am mostly curious about those species that specialize by foraging in the leaf litter of the forest floor regarding the large area that has been flooded," Barrow said. "Millions of trees were toppled in the river bottoms by Katrina, and those that were not felled were stripped of their foliage. Harvey is more of a broad-scale flooding event, so migrants depending on canopy foliage for searching for insects may not be affected that much by Harvey, at least in the greater Houston area."
While a lot of these migrants are insectivorous, many species shift their diet to fruit before heading out across the Gulf because the fruit is higher in lipid content than insects and helps them better replenish their fat. Some fruits the birds typically rely on have dark purple colors that have antioxidant properties and help with oxidative stresses incurred during migration. "So, there is a loss there in terms of nutrition," Barrow added.
Nutrition is important for the flight across the open Gulf, called the trans-Gulf migration, because it can be long. Depending on the route the birds take, their flights can cover as much as 500 to 600 miles and take 18 to 24 hours, Buler said. "There was a study done several years ago tracking gray catbirds and indigo buntings, and they tried to track hummingbirds and some other species," Buler said. "A gray catbird took nine hours. That was the fastest that one of the birds flew from Alabama to the Yucatan Peninsula in the fall."
How humans can help migrating birds
In the short term, the researchers said there will be some mortality from the latest hurricane as well as harm caused by food reductions that may affect breeding next year. But what they really fear with these increasingly intense storms is a shift in the habitat the birds have to adjust to over time.
But Barrow said homeowners can impact that shifting habitat by landscaping with migrants in mind.
"From the 1900s, we have had an incredible recruitment of invasive species in wild and urban spaces," said Barrow, citing a proliferation of the Chinese tallow tree on the western Gulf and non-native species that have proliferated in Florida. Many of these invasive species don't supply the food base that natives do, either because they're new, the insects haven't found them or for other reasons. In addition, invasive species like these disturb habitats.
"We have seen just in the last 15 years a shift on the Louisiana coast from native plants to invasive dominated species because of the disturbance of these storms.
"But because we know from radar observations that these birds are using urban areas in parks, residential green spaces and gardens along the coast, the people who live there can contribute to the birds' journey by using native plants in their gardens and landscape," Barrow says. "It would be especially helpful to the birds for homeowners to choose plants that produce fruits in the autumn or ones with flowers that attract a lot of insects in the spring."
Editor's note: This story has been updated since it was published in September 2017.