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Bird-dar? Watch migration using weather radar.

Updated: Apr 15, 2023

Did you know large movements of birds and other flying animals are captured each year on radar?

Live Migration Map
Bird migration captured on radar in early September around midnight.

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How many birds migrate in North America?

Each year in North America, several BILLION birds migrate north in the spring and south in the fall. How are these numbers determined? Scientists use a tool originally designed for tracking weather events, radar. Utilizing 143 weather stations across the United States, Cornell Lab of Ornithology researchers determined that over 4 billion birds depart the US for Latin America each fall. In the spring, 3.5 billion birds return to the United States.

We've discovered that each autumn, an average of 4 billion birds move south from Canada into the U.S. At the same time, another 4.7 billion birds leave the U.S. over the southern border, heading to the tropics,” noted lead author Adriaan Dokter, an Edward W. Rose postdoctoral fellow at the Cornell Lab. “In the spring, 3.5 billion birds cross back into the U.S. from points south, and 2.6 billion birds return to Canada across the northern U.S. border.

How to read a radar map for bird migration?

Bird Migration occurring in early September
Bird Migration occurring in early September

When looking at a radar map, look first for the obvious storms. These are usually oddly shaped and move from west to east. Next, look for the "biological blooms" or round radar blobs appearing across the map. These blobs usually only begin to appear after sunset at their respective location. There have been instances of meteorologists blowing these blooms off as anomalies, raising the ire of birdwatchers! It even leads to an embarrassing moment for a meteorologist unaware of the extensive studies into bird migration radar. (That poor person had to issue several apologies and retractions. True story.)

Here is what each highlighted area on the radar image above means:

Red - this biological bloom is likely made from birds. The blooms are not always circular, as is the case in the Rockies near Denver, CO.

Green - These are storm systems showing the asymmetrical shape and moving from west to east.

Yellow - This LARGE bloom is likely a combination of birds, insects, and A LOT of bats!


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Want to learn more about bird migration?

Learn more about bird behavior with "What It's Like to be a Bird"

How to use radar to maximize your birdwatching experience?

Using eBird data and the live maps below, you can determine which birds are moving to and from your birding region. This helps some birders find rare species that are pushed off their typical migration course due to winds and storms.

To maximize your experience, follow these steps:

  • Go to

  • Click the 'Explore' tab

  • Select bar charts

  • Select a region or location

  • Use the bar charts to determine the arrival and departure dates of listed species

    • Or click on the Line Graphs icon next to the species name

  • Compare these dates to current migration events shown on the live maps listed below

Learn when large flocks of birds move, like this Snow Goose flock.
Learn when large flocks of birds move, like this Snow Goose flock.

How radar is used for bird conservation?

Nocturnal collisions with buildings are a major ecological hurdle for migratory bird species. There are instances of 500+ birds being killed in a few hours by a single building that failed to turn out its lights during migration events. While shutting out lights every night may not be plausible, researchers have combined radar science, migration statistics, and citizen science data to determine when and where lights need to be out.

CSU and CLO have been able to predict high bird traffic areas and the potential light pollution areas they may encounter. A combination of heavy light pollution and high bird traffic creates a 'high alert lights out warning.' This alerts local governments and property owners that shutting off lights can prevent catastrophic bird kills.


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Live Migration Radar for the United States

The following maps are from the Colorado State AeroEcology Lab, Cornell Lab of Ornithology, and NOAA. A combined effort of these groups created BirdCast, which interprets the radar information, so you do not have to. In fact, these maps can even tell you the direction the birds are moving and how many birds are moving! It is the best tool for watching bird migration by radar.

The following video, from Dr. Kyle Horton, explains how to interpret these base reflectivity maps from NOAA. This timelapse shows these maps' biological and meteorological information throughout a single spring migration.

To learn more about Dr. Horton's work and the projects occurring at CSU, visit their AeroEcology Lab site!

This NOAA Nexrad radar data is combined with wind and velocity data, and scientists create the beautiful bird migration maps seen below.

These images show what the radar station in Houston was showing for bird numbers and movement direction on April 21, 2022, at 3:30 pm. The green in the second image represents birds moving toward the station, and the red is moving away from the station.

This is combined from all stations at a national level, and the results are the maps and forecasts!

What can I use BirdCast for?

BirdCast is a great tool for checking local migration in your region! Want to know if new birds are arriving overnight? BirdCast. Want to forecast which nights will have the most birds moving into or over your area? BirdCast. Want to learn which species likely moved in overnight? BirdCast. Want to learn how many birds crossed over your county last night? BirdCast. Need an alert to know when heavy migration is coming through (so you can shut off your darn lights)? BIRDCAST!!!

An example of the information BirdCast provides.
An example of the information BirdCast provides. This data is from 4/11/2023 across the state of Texas.

BirdCast is the most complete bird migration prediction, alert, and reporting entity available in North America. However, one of the founding minds behind BirdCast, Dr. Kyle Horton, also has an amazing site with an extension of this amazing information. Make sure to visit both sites and support both groups to help keep this amazing science going forward!

What can you use BirdCast for? Everything. (Except for cooking dinner. That is still on you. Or McDonald's. Or whatever poor sap is making your food.)


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Bird migration is a magical time of year. Scientists are capturing some of that magic using an unusual system, weather radar. Using this amazing tool, along with community science efforts, we have a system that can predict birds as well (or better) than that next drizzle! Just don't yell at the "bird man" on TV if they get it wrong.


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