From the Autumn 2018 issue of Living Bird magazine. Subscribe now.

Consider the sky. Between the treetops and the moon lies a layer cake of opportunities for a migrating bird. We can’t see them or feel them, but they are there: bands of warm and cool air; headwinds, tailwinds, crosswinds. Mist, fog, clouds, rain. Winds that push migrants on course or off, or sweep insects together like a broom.

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To put it simply, air is habitat. But it’s only recently that ornithologists—after generations of cataloging earth and ocean into ever-finer habitat types— have begun to consider it as such.

“It probably bucks what most people think of as a habitat,” says Kyle Horton, a postdoctoral researcher at the Cornell Lab of Ornithology. “It’s not land and it’s not water, but it’s still something that birds make use of.”

Historically, Horton says, we’ve thought of the air as something birds move through between habitats. As an example, check what a bird guide like All About Birds has to say about aerial species like Purple Martins: Habitat type—towns, cities, forest edges, ponds, marshes. Not air.

But that’s changing. In 2013, U.S. Geological Survey ecologist Robert Diehl called into question this reluctance to accept airspace as habitat. Sure, it’s invisible to us, he noted, but it’s still a swirling mixture of wind, temperature, precipitation, light, and magnetic field, with just as much potential to concentrate resources as any ocean current. He closed his paper with a sly parting shot: “Either the airspace is habitat or flying animals are exceptional for their ability to be outside it.”

This reassessment is not limited to aerialists like swooping swallows and skydiving swifts. Even birds that seem positively earthbound, like Ovenbirds and Grasshopper Sparrows, become connoisseurs of the atmosphere twice a year during migration.

A recent study led by Horton and published in the journal Biology Letters is helping to sketch out the main features of this new realm, and in the process offering insights into the hazards birds face in the air. The 2016 study took advantage of a new generation of high-resolution radar stations that can estimate the numbers of birds in the air, their altitude, speed and direction over the ground, and even the heading, or direction the flying bird is pointed.

By comparing three inland radar stations with three radars along the Atlantic coastline, the study found that birds routinely seek out helpful winds. They wait until a favorable weather system arrives and, once aloft, seek out atmospheric layers with better winds.

For a migrating male songbird in spring, making the right choice could mean the difference between arriving early to get first pick of breeding territories or showing up late and being stuck with leftovers. For a female, it could affect how much energy she has left for laying eggs. Horton and his colleagues measured the so-called “wind profit” that birds gained by selecting favorable altitudes. They determined that spring migrants flew on average 15 miles per hour faster than they would have in still air—almost twice as fast as a bird’s typical flight speed without help from the wind.

“If they can double their distance by selecting the right height layer, I think that has a strong evolutionary pressure” on birds to key in on specific parts of the atmosphere, Horton says. “There are so many examples of how winds have shaped migration.”

Bird watchers may not think of air as “good habitat” or “bad habitat” in the way they might size up an old field or a patch of second-growth forest. But favorable air is the reason why North America’s Central Flyway is the “habitat” chosen by hundreds of millions of birds for their migratory flights. “Not a lot of [forest] birds breed along the Central Flyway,” Horton says, “but that’s the route they take to the boreal forest, because that’s where the winds are most favorable.”

The same goes for why songbirds migrate at night: the “habitat quality” of the air is better, because the air cools and turbulence—dangerous to a half-ounce songbird—subsides.

radar-generated map forecast of bird migration intensityEstimates of migration activity from our BirdCast project show how birds respond to atmospheric conditions and weather systems when they migrate. Image courtesy BirdCast project.

Despite their savvy, birds are still at the mercy of strong winds when they blow in the wrong direction. That’s perhaps one reason behind a long-standing pattern in which young-of-the-year songbirds concentrate along the coast of the eastern U.S. during fall migration.

“Look on any weather map. The thunderstorms almost always move from the west to the east,” Horton says. “If you dropped [any kind of] particle in the airspace, it would get blown to the coast.” For instance, a 1981 study noted that 98 percent of Palm Warblers caught at coastal banding stations in the fall are hatch-year birds. Fifteen other warbler species show a similar pattern, with young-of-the-year birds making up more than 90 percent of coastal captures.

These first-time migrants may wind up at the coast because they’re less adept than adults at navigating an airspace habitat of unfavorable winds. Radar data is also helping to put more precise numbers on how high birds fly, and it’s not as high as once thought. Before radar became widespread, height records were opportunistic, giving unwarranted weight to extreme sightings. For example, a Mallard once collided with an airplane at 21,000 feet, setting a height record for the species, even though most waterfowl migration happens below 5,000 feet.

Horton says small birds tend to fly the lowest, with waterfowl, shorebirds, and herons flying the highest. Raptors tend to fly relatively low (below 3,000 feet) to take advantage of thermals.

But “most birds are almost always below 500 meters [1,640 feet],” Horton says. “Many people think birds fly super high, and 500 meters is still high, but it’s fairly close to the surface of the earth.”

Bird mortality estimate sources: SR Loss, et al. (2012). Direct human-caused mortality of birds. Annual Review of Ecology, Evolution, and Systematics 46: 99-120. Wind turbine estimate—SR Loss, pers. comm. Migration elevation sources: waterfowl—Ducks Unlimited; raptors—P Kerlinger, et al. (1985). Flight behavior of raptors during spring migration in South Texas studied with radar and visual observations. Journal of Field Ornithology 56: 494-502. songbirds—KP Able. (1970). A radar study of the altitude of nocturnal passerine migration. Bird-Banding 41: 282-290. Graphic by Bartels Science Illustrator Megan BishopMost birds migrate relatively low to the ground, putting them at risk of hitting human-made structures. Deaths from collisions with various structures were estimated by Loss et al. (2012), Annual Review of Ecology, Evolution, and Systematics. Migration elevations from Ducks Unlimited, Kerlinger et al. (1985) Journal of Field Ornithology; Able 1970 Bird-Banding. View larger image. Graphic by Bartels Science Illustrator Megan Bishop.

And unfortunately, it’s where virtually all the human-caused hazards are. For example, a bird flying at 500 meters still wouldn’t clear the needle on top of Chicago’s Willis Tower (formerly known as the Sears Tower), the second-tallest building in the United States. Combine buildings with the disorienting effects of lights, and the night sky becomes a forest of dangers for migrants, luring them toward windows, causing them to waste time and energy flying off course, or just pulling them down into an urban habitat that may not have enough food for the bird to refuel.

That’s why organizations like Toronto’s Fatal Light Awareness Program work to get unneeded lights turned off in the city during migration. Tall buildings can kill tragic numbers of birds in an evening. And single-family homes, because of their sheer number, kill many millions of birds each year due to collisions.

To Horton, the difficulties that migratory birds face as they navigate their aerial habitat are at least as compelling as dangers in their breeding or wintering grounds. He dares to imagine a day when key parcels of airspace are protected as lands or waters are now.

“It’s kind of a pie-in-the-sky idea,” he says. “Let’s box out a national park that’s an airspace. No lights, no airplanes, no drones, no wind turbines, no cell towers, no buildings. It seems ludicrous, but if that’s where a lot of birds are, then it’s not any different than saving a grassland.”