4 hours ago

So many birds are migrating that they’re appearing on weather radar

Hundreds of millions of birds are taking to the skies each night, southward-bound in search of warmer weather - so many that you can spot them on weather radar. It’s data that researchers can use to estimate how many birds are in the skies and where they’re going.

The Cornell Lab of Ornithology processes such data, which is taken from the National Weather Service’s network of 159 specific weather surveillance doppler radars scattered about the country. Weather-related signals are filtered out and the data is analyzed, leaving scientists with a nightly bird count of sorts.

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The project has been ongoing in some capacities since 2000, but its accuracy has grown over time in tandem with advancements in the fields of radar technology, machine learning and signal processing.

Findings are made publicly available on a site called BirdCast. Each day, the site publishes a high-resolution recap of how many birds were in the sky the preceding night, broken into 10-minute intervals. But with 20 years of data saved and with a knowledge of weather and migratory patterns, the agency can also forecast bird migration numbers up to two weeks in advance.

So how does it work? Simple: weather radars can essentially see whatever is in the air - whether that be rain, snow, animals or even meteors. The more material in the atmosphere, the greater the amount of signal that’s bounced back to the radar. (Imagine standing in your backyard and yelling - you’d hear a greater echo off an oversize plywood wall than you would a chain-link fence. Weather radars hear louder “echoes” when there are more objects to reflect the radar beam.)

Conventional weather radars rotate, scanning the skies by emitting pulses of electromagnetic radiation and then listening for echoes. Over a total hour, the radar only actually transmits electromagnetic waves for a total of 7 seconds; the remaining 59 minutes and 53 seconds are spent in listening mode.

Between 2010 and 2013, the radars were upgraded with technology that allows both horizontal and vertical pulses of energy to be emitted. By comparing the returned signals, meteorologists can determine the shape of whatever is in the sky. Raindrops are a bit wider than they are tall, and shaped like hamburger buns; snowflakes are - obviously - flaky; but lofted tornado debris is spiked or jagged.

Birds, meanwhile, appear as somewhat spiked objects, as do insects. But insects appear a bit more round and uniform on radar, and are also lightweight enough to become caught up in the wind. Birds travel higher than most bugs, and also can fly against or perpendicular to the wind. After all, they have places to go - southward. Meteorologists can also determine their direction of motion through their analyses.

In the coming weeks, you’ll probably continue to see flocks of birds taking flight around or just after sunset. And when darkness falls, you can keep looking out for them - on weather radar. Researchers encourage citizens to dim or extinguish outdoor lights overnight, particularly between the hours of 11 p.m. and 6 a.m. That will reduce the number of bird casualties and collisions; artificial lights are also known to interfere with birds’ navigation.