If you step outside tonight and look up at the sky, you probably won’t see much happening. But high above you, millions of birds could be making one of the most remarkable journeys in the natural world.

Fall migration is getting underway across North America, and birds are beginning their annual journeys south toward their wintering grounds. The remarkable part is that much of this migration happens after dark, when the birds are almost completely invisible to us.

According to BirdCast, more than 80% of migratory bird species in North America migrate at night. The project uses weather surveillance radar across the United States to track these movements, allowing scientists to estimate how many birds are in the air, where they’re heading, and how quickly they’re moving.

The timing is particularly interesting right now, as BirdCast identifies mid-August through mid-November as the main period for nocturnal fall migration in North America – although the peak varies considerably depending on location.

BirdCast, live migration map (US); 17th of August 2026: 00:10 ET. Cornell Lab of Ornithology.

Where Are The Birds Going?

The majority of birds beginning their fall migration are heading south, although exactly where they travel depends on the species.

Some birds will make relatively short journeys, while others will travel thousands of miles to reach their wintering grounds in the southern United States, Mexico, Central America, the Caribbean and South America.

The United States sits directly beneath several major migration routes, or flyways as they’re otherwise known. These include the Atlantic Flyway along the eastern seaboard, the Mississippi Flyway through the center of the country, the Central Flyway through the Great Plains, and the Pacific Flyway in the west.

Flyways in North America (Wikipedia)

That means the number of birds passing overhead can vary enormously depending on where you live.

BirdCast’s Migration Dashboard allows people to check estimated migration activity for individual counties across the United States. It provides information including the estimated number of birds in flight, their direction of travel, speed, and altitude.

The birds aren’t simply following a fixed route, either. Weather plays an important role in determining when migration takes place. Birds can take advantage of favorable winds to make their journeys more efficient, while strong opposing winds or bad weather can cause them to delay their departure or stop along the way.

Why Are Birds Flying At Night?

There are several advantages to migrating after sunset. The cooler nighttime temperatures can help birds avoid some of the heat and energy demands associated with flying during the day. Many migrating species also take advantage of favorable winds higher in the atmosphere.

For birdwatchers, however, the nighttime migration creates an intriguing problem – as you can’t see it happening.

That’s where weather radar comes in. BirdCast’s live migration maps use data from the US weather surveillance radar network to detect birds moving through the atmosphere between sunset and sunrise. The technology can reveal huge movements that would otherwise be almost impossible to observe.

In September 2025, for example, BirdCast detected more than 1.2 billion birds migrating south during a single night, setting a record for the largest single-night migration total recorded by the project since its live mapping began in 2018.

That doesn’t mean every night produces numbers anywhere near that level. Migration intensity changes dramatically depending on the weather, time of year, and location. But it certainly gives us an idea of just how much wildlife can be moving overhead while most people are asleep.

Scientists Can Now Work Out Which Birds Are Migrating

In 2026, researchers from the Cornell Lab of Ornithology, the University of Massachusetts Amherst, and the University of Illinois developed new methods for estimating which individual bird species are responsible for migration detected by radar.

The research combines weather-radar data with more than 2 billion bird observations submitted by citizen scientists through eBird. This is significant because radar can tell researchers that birds are flying overhead, but traditionally it hasn’t been able to tell them exactly which species are making that journey.

The new BirdFlow Migration Traffic Rate model can produce species-specific estimates of migration across North America and provide weekly estimates of where different species are likely to be moving.

That could eventually give birdwatchers a much clearer picture of not just how many birds are migrating over their area, but which birds are making the journey.

For now, if you want to know what’s happening above your own backyard tonight, the best place to start is BirdCast’s live migration forecast.