In the video above, posted a couple years back by Christopher Wood, blue stippled circles bloom and spread over the continental U.S., all coinciding with the half-hour after local sunsets. Those circles are birds—this is a pattern typical of large groups of birds taking off to resume migration.

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Weather radar can tell you whether to pack an umbrella and poncho for the day, but, to ornithologists and others in the know, it also reveals the tell-tale signatures of migrating birds. The radio waves sent out by doppler radar bounce off raindrops and birds alike and return a signal to the receiver, writes Hannah Waters on her Scientific American blog.

Birds aren’t the only flying critters visible on radar. Last week, migrating monarch butterflies winging their way through St. Louis created blue and green splotches in the radar picture on a sunny, clear day. On Facebook, the U.S. National Weather Service St. Louis wrote: "A Monarch in flight would look oblate to the radar, and flapping wings would account for the changing shape!"

Keen observers can distinguish between bird signals and non-bird signals using several bits of information. First, birds typically show up as those growing concentric circles. Also, the time of day is important. Many species of migrating birds take off in the evening, fly through the first part of the night and land by sunrise. David A. La Puma, the director of the Cape May Bird Observatory, gives more tips on his website woodcreeper.com:

Using the velocity image, though, you can get an idea of how much non-bird noise you’re seeing on the reflectivity image. Birds tend to migrate 15-20 kts [knots] faster than the prevailing wind (given the wind is a tailwind), so by 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.

Those slow moving signals are likely to be insects, which travel with the wind.

Using radar to track the movements of birds, bats and insects isn’t new—researchers realized that animals were visible on radar during World War II—but the Internet now gives interested lay people a chance to see the phenomenon for themselves.