Fall migration maps reveal where birds are flying over North America

Fall migration maps reveal where birds are flying over North America

If you’re like me, you’re probably easily fed up with the weather forecast. It’s often wrong or shows me what I don’t want to see.

I’m here to tell you that there’s a better forecast out there — a forecast for birds.

Called BirdCast, the forecast provides a three-day outlook of how many birds will be flying overhead across the country — and where. After sunset on Tuesday, for example, BirdCast estimates that 74,000 of them will journey through the skies of New York City. The tool, which was developed by the Cornell Lab of Ornithology and a handful of other universities, also shows where large numbers of birds are cruising through the sky in close to real time. In other words: It’s radar, but for birds.

This kind of predictive technology is especially useful as fall migration nears its peak in North America. Each autumn, billions of migratory birds travel south in search of food and warmer weather, some flying several thousand miles. The famous Arctic tern makes an epic journey of more than 12,000 miles from the Arctic to Antarctica — a distance that surpasses the world’s longest nonstop commercial flight.

Migration offers something of a fleeting backyard paradise even to novice birdwatchers. You can see all kinds of unique species in your local patch of green as they’re passing through, such as warblers, thrushes, and sparrows in New York City’s Central Park. Forecasting maps like BirdCast help by revealing migration hot spots, from southern Texas to the Great Lakes region.

These tools are also a boon for birds. Hundreds of millions of them die each year from colliding with windows, and disorienting lights are a big part of the problem. Forecasts can help cities pinpoint when to turn off the lights, allowing more of the winged passersby safe passage.

Chicago’s Field Museum has collected tens of thousands of birds that collided with buildings downtown. These four are from its collection.
Courtesy of the Field Museum

See where birds are flying overhead

Our ability to map and forecast birds is rooted, like so many innovations, in war. Engineers developed radar technology during World War II to detect enemy aircraft, which basically involves broadcasting microwaves and seeing what they bounce off of — not unlike how a bat might use sonar to map out a dark cave. During the war, radar operators began noticing strange dots — known as “angels” — appearing on their screens. The operators, some of whom were birders, eventually figured out that the dots weren’t bomb-carrying planes at all. They were birds.

The discovery sparked a revolution in ornithology. In the eight decades since, scientists have used radar technology to detect birds and swarms of insects in the night sky, just as meteorologists have used it to map out hurricanes and rainstorms. More recently, major advances in computing have made it easier to process large amounts of data, allowing scientists to put together detailed migration forecasts in a short period, said Andrew Farnsworth, a senior research associate at the Cornell Lab of Ornithology, who leads the BirdCast project. We’re now in the era of radar ornithology, he said.

BirdCast’s live map shows where birds are migrating. Warmer colors indicate higher numbers of birds are passing through an area.
Van Doren and Horton/BirdCast

First launched in 2000, BirdCast runs on software that analyzes weather radar to discern what’s a bird versus a cloud or another object. That analysis produces a map of the US that shows where birds are migrating in the sky, like the one above, with warmer (oranger) regions showing where there’s more bird traffic — that is, more birds moving through an area at a given time. Pretty neat, right?

BirdCast also produces three-day forecasts, which estimate nighttime movements because the vast majority of North American birds migrate after dark. Researchers have a good idea of how weather affects their journeys — temperature, which affects the speed and direction of wind, is especially important for determining when birds travel, Farnsworth said. By leveraging weather forecasts, BirdCast estimates where birds might migrate in the near future. (You can find the forecast and “live” migration maps here.)

BirdCast’s migration forecast for Thursday, September 30.
Van Doren and Horton/BirdCast

You can use these maps to go birdwatching, though keep in mind that they show the number of birds in the sky — not on the ground — and that the forecast is for nighttime migration. Where the predictive power really comes into play is in helping cities, big and small, reduce the number of birds that die from smashing into buildings.

How bird forecasts can cut down on window collisions

Each year in the US, between 365 million and 988 million birds die from running into buildings, researchers estimate. It’s the second-leading cause of avian death in North America after predation by house cats, and light pollution is a big part of the problem. Lights can attract and disorient birds, causing them to crash.

That’s what makes bird forecasts so useful: When we know a large number of birds are set to pass through a particular region, it’s clear that we can prevent strikes by turning off at least some of the lights in buildings at night. It’s really that simple — and we know it works.

One study published this summer, for example, found that halving the area of windows that are illuminated at a large convention center in Chicago during spring migration led to 11 times fewer collisions. The authors further concluded that dimming lights during migration could reduce the number of collisions at the center by about 60 percent. You rarely come across wins of that magnitude in conservation — and literally overnight, at that.

BirdCast has a feature called “lights out alerts,” which reveal which cities should prioritize turning out the lights, and when.
Van Doren and Horton/BirdCast

Across the US, more than 40 cities now have Lights Out programs, and some, including New York, have passed legislation requiring that buildings be more bird-friendly — such as by using glass with patterns that make windows more visible to birds. BirdCast, for its part, has a feature called “lights out alerts” (shown above) that can help cities figure out when to go dark.

Similarly, researchers have proposed using forecasts to help reduce bird strikes at airports, which cause more than $1 billion in damage each year globally. In one recent study, scientists analyzed plane-bird collisions at three New York City airports and found that weather radar can “accurately predict the probability of bird strikes.” Theoretically, airports could use that information to make flying safer and less costly.

What radar is revealing about birds

Bird radar isn’t just useful for birdwatchers and collision prevention. It’s also helping scientists answer some of ornithology’s biggest questions, such as how climate change is affecting the timing of migration.

Results from a radar-based 2019 study, for example, found that rising temperatures are shifting spring migration about two days earlier per decade in the US, on average. (The changes are much greater as you move farther north.) That may not sound like much, but migrations are carefully planned events where timing is a matter of life and death.

Birds have adapted to arrive at particular locations at particular times — for example, “when the pulses of spring insect blooms are peaking,” Farnsworth said. “With earlier rising temperatures, birds may arrive after the pulse of food availability, creating a growing mismatch. If this happens rapidly, bird populations may not be able to keep up.”

Researchers are also using radar to more closely investigate some of the unknowns of migration, like what happens to birds during a big thunderstorm or when wind patterns suddenly change. They previously thought storms might bring migration to a halt, Farnsworth said, but radar suggests otherwise. “The thought that rain automatically stops migration is totally wrong,” he said. “Birds are moving in all kinds of conditions.”

Yet despite all of the advances in radar ornithology, one major mystery remains largely unsolved. How do birds navigate, anyway? I still get turned around in my hometown, so it’s hard to imagine that birds can travel thousands of miles to a destination they’ve never seen or visited. To pull off such a feat, they likely use some combination of the sun, the stars, Earth’s magnetic field, and possibly even their sense of smell. But exactly how they piece all that information together to navigate is “a total mystery,” Farnsworth said. “I don’t think we’re anywhere near beginning to understand it.”