Several times a week, if not every day, I look at Doppler radar maps so I know whether to take an umbrella when I leave the house. These maps, shown on TV weather reports or websites, are commonplace enough that they don't feel like impressive technology: mere green blobs slowly shifting across the screen at worse resolution than most animated gifs. They are so unremarkable that I've never taken a moment to consider how they work—or whether they could pick up data besides the chance of rain.

That is, until my dad posted this image of NJ Doppler radar on my Facebook wall last week. The blue circles, indicated by arrows, are not rain, but flocks of birds taking off. From the Philadelphia/Mount Holly National Weather Service:

This is not precipitation, but it is large gatherings of birds that are taking off to set out for the day. There are so many birds that the radar beam is partially returned showing the signatures below. We see this several times a year on the Doppler radars right around sunrise. A radar loop would actually show the circular pattern becoming larger with time before dissipating as the birds fly up and outward.

NWS meteorologist Michael Gorse told me by email that these birds were likely swallows and martins, based on a report from the day before along the New Jersey Shore. "Many bird enthusiasts utilize radar imagery to track migration patterns," he wrote. "Radar imagery has also been helpful to both birds and humans regarding aviation safety. Most airport terminals use radar data to track birds as they cross flight paths to avoid collisions."

Doppler radar works by sending out a radio wave pulse and then "listening" for its return signal when it bounces off an object in its path. When the pulse strikes an object—a bird, a bug, a raindrop—the energy scatters, and a small fraction returns to the radar. Computers analyze the strength, time and direction of the returned energy to calculate the size, distance and direction of the object. The U.S.'s 155 WSR-88D Doppler radar stations do this up to 1,300 times every second, and all the information is available online.

The most difficult part of using this for birdwatching would seem to be separating the normal rain signals from those of animals. This can be done using basic knowledge of the migratory behavior of birds. David A. La Puma, director of Audubon's Cape May Observatory in NJ and author of the now-defunct radar ornithology blog Woodcreeper, wrote on his blog's FAQ that migratory birds tend to fly at night. They take off at sunset, close to the grounded Doppler radar stations, and as they fly upwards, the signals appear further and further away. This creates a concentric circle-like pattern on the map readout that, when animated, looks rather spectacular, like fireworks:

Sometimes other flying animals, like insects, can create a similar pattern if there are enough of them. But, as La Puma explained on his blog, you can tell the difference based on flying behavior. Birds tend to fly faster than the prevailing wind, while insects, pollen and other obscuring objects tend to travel at the same speed. "[B]y checking the winds at 950mb (2500 feet, roughly), you can determine the wind speed, compare it to the speed at which the objects are moving across the radar, and therefore rule out any object traveling too slow to be birds," he wrote.

Distinguishing birds and other animals from known patterns on radar has been a problem since World War II, when radar was used to home in on enemy warcraft. Then they were confused with fighter planes occasionally, but more frequently with ships. "Birds not infrequently travel with a ground-speed similar to that of a fast-moving ship, and at long range the echo from a bird flying fully in the beam of a radar set can be equal in strength to that from a ship which is below the lowest maximum of the vertical polar diagram," wrote ornithologist David Lack and entomologist G. C. Varley in a 1945 letter to Nature. "Birds have given rise to several E-boat scares and to at least one invasion alarm." They caused enough unnecessary military panic that some radar operators received special training to distinguish them from warcraft.

Lack and Varley reported that they were able to confirm that the radar readings were birds by two methods. In the first, Major J. A. Ramsay of Coast and Anti-Aircraft Experimental Establishment suspended a herring gull (presumably a dead one, rather morbidly) from a weather balloon and then observed the radar signal. He was able to distinguish between the balloon and the bird, indicating that birds are big enough to picked up by radar. In the second, grey geese were identified by their calls and then tracked from one radar station to the next. "This record constitutes much the longest timed track so far available for any bird in flight," wrote Lack and Varley.

Since, radar ornithology has been used to study patterns of bird flight for conservation purposes, but rarely without the help of observant people on the ground to confirm bird species and numbers. In 1985, Sidney Gauthreaux, director of the Clemson University Radar Ornithology Lab and pioneer of the technique, built a mobile radar lab that, along with binocular observation, allowed him to study how birds interact with electrical lines (PDF). Now he's using it to track flocks of migratory birds in Texas to discover important conservation areas.

Radar aeroecology isn't limited to birds, however. It can be used to count and track bats to find out whether populations are thriving or on the decline. It can do the same for insects, such as when a swarm of migrating monarchs was picked up by the NWS in St. Louis, Missouri last week. And now that radar images are readily available online, dedicated birdwatchers can use them to track migratory birds in the spring and fall to pick the best mornings to wake up early to peer through their binoculars.

And for the rest of us casual observers, we can admire the beautiful images produced by birds on radar. This animated map blooms at around 8pm (0:13) as flocks take off across the United States for their nocturnal flight.

I don't know about you, but I will never look at weather radar the same way again.

Animated map by Michael Kurz, NWS Wilmington, DE