Every fall, millions of birds take to the skies across the United States after sunset, beginning journeys that can stretch thousands of miles. But with darkness blocking our view of the birds flying hundreds – if not thousands – of metres in the air, how do ornithologists track them?

Luckily, the answer is fairly simple – and it’s one that stretches back to the 1940s. 

Image Credit: Dibert

Ornithologists track birds via high-end weather radar systems

Ornithologists have been tracking bird migration across North America with weather radar systems since World War II. Back then, British radar operators noticed strange echoes on their screens that didn’t correlate with aircraft or weather. At the time, they were nicknamed “angels”. Over time, researchers established that the echoes on their screens were, in fact, being caused by birds. 

Fast forward to today, and ornithologists use the latest weather surveillance radar networks – specifically the NEXRAD network – otherwise known as the WSR-88D. It’s the same surveillance system that BitrdCast uses to highlight migrating birds on its heatmap. 

BirdCast, live migration map (US); 26th of August 2026: 06:00 ET. Cornell Lab of Ornithology.

As birds take off after sunset, they can appear on radar as areas of movement that spread across the landscape. Researchers can analyze these signals to estimate how many birds are migrating, where they’re heading, and how intense the migration is. This technology has transformed what scientists know about nocturnal migration.

BirdCast, a collaboration involving the Cornell Lab of Ornithology, Colorado State University, and the University of Delaware, uses weather surveillance radar to monitor bird migration across the Lower 48 states. The system can even produce forecasts showing where major migration events are expected to occur.

Why do birds migrate at night?

For many migratory birds, nighttime is the perfect time to travel. The air is generally cooler; there can be fewer predators, and birds can spend daylight hours searching for food and recovering from their journeys.

That doesn’t mean every bird migrates at night. Many larger species, including hawks and other soaring birds, tend to migrate during daylight. But huge numbers of songbirds make their journeys after dark. And that’s where radar becomes incredibly useful.

What does a radar image of birds actually look like?

When migration is particularly heavy, radar can show a distinctive pattern as birds leave the ground shortly after sunset (see image above). Scientists can then separate biological activity from weather and other sources of radar returns. The data can reveal just how enormous these migrations really are.

On a favorable autumn night, more than one billion birds can be in the air over the United States at the same time. That means there could be thousands of birds flying over a single town while the people below are completely unaware.

Scientists can now predict migration before it happens

Radar isn’t only useful for looking at migration after it has happened – but also gives us a glimpse into when migration could take place. BirdCast combines radar observations with weather information to forecast when and where major migration events are likely to occur.

Birds can use favorable winds to reduce the amount of energy required to travel, so large movements often occur when weather conditions provide a useful tailwind. That means scientists can sometimes warn birdwatchers – and cities – that a major migration event is coming.

During heavy migration periods, conservation groups can encourage buildings and homeowners to reduce unnecessary nighttime lighting, which can attract and disorient migrating birds.