Birds and hurricanes have always co-existed in 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 2017 will be especially treacherous, especially for migratory land birds on their journey from breeding grounds in North America to winter homes in the tropics. That’s because two of the most powerful and devastating storms ever recorded have impacted the birds’ eastern flyway, the path that takes them through Florida, and their central flyway through Alabama, Louisiana, Mississippi and Texas.
The effects of these hurricanes on current migration patterns are being watched closely by a group of researchers who have launched a project 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 the most recent storms are modifying birds’ migratory movements — and they may even be able to do that in real time with Irma.
"We might be able to say something about the impact of Irma as it moves through Florida," says Jeff Buler, an associate professor of wildlife ecology in the Department of Entomology and Wildlife Ecology at the University of Delaware. New Doppler weather radar technology gives them that capability because it distinguishes what he calls bioscatter, animals that the radar detects and distinguishes from precipitation. Even with this advanced technology, though, they won’t be able to determine how many birds might have been killed by the force of the winds or were carried out to sea and drowned. That sort of information, Buler says, would require telemetry tags on specific populations of birds.
With the substantial information they've been able to accumulate on both storms, though, as well as data from previous hurricanes such as Katrina and Super Storm Sandy, here is their assessment of how Irma and Harvey will impact the 2017 fall migration.
Impacts of 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)
Songbirds affected by Hurricane Irma are traveling the Eastern flyway on a route that takes them through Florida and then across the Caribbean and into Central and South America. "These birds are very generally thrushes, warblers, flycatchers and sparrows," says Buler. This 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 says. 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. ( In this paper, Frank La Sorte, a macroecologist at the Cornell Lab of Ornithology, offers a more complete explanation of loop and other migrations. And for an up-to-date look at what birds are moving through this flyway as well as other flyways, visit the regional forecasts site provided by the Cornell University Lab of Ornithology in Ithaca, New York.)
This fall, though, the birds face a dual threat during the height of the fall migration in September from the brunt force of Irma’s winds, Buler says. One threat is the loss of food resources, like insects or fruiting fall flowers that have been stripped of vegetation. The other is the possibility of birds being carried off course by the storm, perhaps even back to the starting point of their migration! And on the heels of Irma, birds migrating over the Atlantic are facing another threat in Hurricane Jose.
One way birds can be carried off course is through a phenomenon that Buler calls being entrained 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 will seek shelter in the eye and just keep flying inside the eye until the storm passes over the coast where they will 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 are not supposed to be.
Charlotte Wainwright, who lives in England, was one of the birders who followed Irma’s impact on seabirds as the storm crossed the Atlantic. She did that from her home in the United Kingdom by following radar that was detecting birds over the ocean at 1.6 kilometers up in the air and then tweeting out her findings.
Another possible 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, will be monitoring 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, the 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.
The BirdCast is also tracking Irma’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."
Impacts of Hurricane Harvey
As with Hurricane Irma, 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," says Barrow. "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 adds. "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 says, 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 continues. "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."
Weeks after Harvey, researchers still have not been able to fully assess how migratory land birds stopping over in the Houston area have responded to the hurricane’s devastation there. But they do know that birds outside of the city will have less-than-ideal nutrition choices. To the east and north of Houston, for example, are not-so-desirable piney woods. To the west beyond the Colorado river there is less forest cover.
"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," says Barrow. "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 that 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 adds.
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, says Buler. "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 adds. "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."
No one knows for sure what will happen if the birds can’t fatten up due to a lack of food resources. Will they stay in the northern Gulf region for the winter or continue their journey in a weakened state? Barrow suspects they will continue their migration to their wintering grounds to the extent they can. "But they will do that in a condition that is probably less than optimal."
But humans can help
In the short term, Barrow says he and Buler know there will be some mortality from Harvey and Irma 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 wants homeowners to know there’s a way they can impact that shifting habitat, and he has a suggestion for that: Landscape with migrants in mind.
"From the 1900s, we have had an incredible recruitment of invasive species in wild and urban spaces," says 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 do not 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."