In late April of this year, a storm system spawned 84 confirmed tornadoes that swept through the central and southern U.S. Now, researchers show that songbirds knew in advance about what was coming and flew the coop before the deadly supercell storms blew in. This demonstration of nature’s early-warning system was published in Current Biology this week.

Birds have been known to alter their regular migration route in order to detour around potentially injurious events. “But it hadn’t been shown until our study that they would leave once the migration is over and they’d established their breeding territory to escape severe weather,” UC Berkeley’s Henry Streby says in a university statement.

Streby and colleagues didn’t set out to document storm avoidance behavior. They were testing half-gram geolocators on golden-winged warblers (Vermivora chrysoptera)—who weigh less than two nickels—when the birds abruptly took off from their breeding grounds in the Cumberland Mountains of eastern Tennessee (pictured below). The warblers spend winter in Central and South America before returning to North America’s Great Lakes and Appalachian Mountain regions in the spring to breed. They had only just arrived before skipping town for this unplanned trip.

"The most curious finding is that the birds left long before the storm arrived," Streby says in a Cell Press release. More than 24 hours before, and while the storm was between 400 and 900 kilometers away. "At the same time that meteorologists on The Weather Channel were telling us this storm was headed in our direction, the birds were apparently already packing their bags and evacuating the area."

Based on five of the geolocaters, the warblers ended up traveling 1,500 kilometers in five days to avoid the storm, and they came right back to their established breeding territories after the storm passed. They all flew in different directions, and one had fled as far as Cuba. Pictured to the right, you can see a geolocator (black with a white light sensor) on the back of a male golden-winged warbler.

Wind, atmospheric pressure, temperature, and rainfall didn’t tip them off—these hadn’t changed all that much yet. But tornadic storms make very strong infrasound that travel thousands of kilometers ahead of the storm. And at below 20 hertz, these are well out of the range of what we can hear, but in exactly the same frequency that birds are most sensitive to hearing. Sound waves with lower frequencies travel the farthest.

"Our observation suggests [that] birds aren't just going to sit there and take it with regards to climate change, and maybe they will fare better than some have predicted," Streby says. "On the other hand, this behavior presumably costs the birds some serious energy and time they should be spending on reproducing."

Images: Henry Streby and Gunnar Kramer