Birds sensed severe storms and fled before tornado outbreak

Golden-winged warblers apparently knew in advance that a storm that would spawn 84 confirmed tornadoes and kill at least 35 people last spring was coming, according to a report in the Cell Press journal Current Biology on December 18. The birds left the scene well before devastating supercell storms blew in.

The discovery was made quite by accident while researchers were testing whether the warblers, which weigh "less than two nickels," could carry geolocators on their backs. It turns out they can, and much more. With a big storm brewing, the birds took off from their breeding ground in the Cumberland Mountains of eastern Tennessee, where they had only just arrived, for an unplanned migratory event. All told, the warblers travelled 1,500 kilometers in 5 days to avoid the historic tornado-producing storms.

"The most curious finding is that the birds left long before the storm arrived," says Henry Streby of the University of California, Berkeley. "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."

The birds fled from their breeding territories more than 24 hours before the arrival of the storm, Streby and his colleagues report. The researchers suspect that the birds did it by listening to infrasound associated with the severe weather, at a level well below the range of human hearing.

"Meteorologists and physicists have known for decades that tornadic storms make very strong infrasound that can travel thousands of kilometers from the storm," Streby explains. While the birds might pick up on some other cue, he adds, the infrasound from severe storms travels at exactly the same frequency the birds are most sensitive to hearing.

The findings show that birds that follow annual migratory routes can also take off on unplanned trips at other times of the year when conditions require it. That's probably good news for birds, as climate change is expected to produce storms that are both stronger and more frequent. But there surely must be a downside as well, the researchers say.

"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." The birds' energy-draining journey is just one more pressure human activities are putting on migratory songbirds.

In the coming year, Streby's team will deploy hundreds of geolocators on the golden-winged warblers and related species across their entire breeding range to find out where they spend the winter and how they get there and back.

"I can't say I'm hoping for another severe tornado outbreak," Streby says, "but I am eager to see what unpredictable things happen this time."

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Breaking Down the March Tornado Outbreak

New images from the nation's Storm Prediction Center show how much of the South and Midwest were affected by the severe weather and tornadoes on Friday (March 2).

An entire month's worth of tornadoes struck across parts of the country from March 2 into the morning of March 3. The Storm Prediction Center received 81 reports of tornadoes on March 2, according to data filtered to remove duplicate reports of tornadoes. For the entire month of March, the 10-year average number of tornadoes is 87, according to the Weather Channel's severe weather expert Greg Forbes. The National Weather Service's storm survey teams have not yet confirmed the tornado reports, so these numbers could change. But if the numbers hold, the outbreak could go down as the largest single-day outbreak in March history.

In 2006, the biggest March outbreak saw 105 tornadoes from March 9 to 13. March 12 of that outbreak saw 62 confirmed tornadoes. Yesterday's outbreak could top that total.

The unfiltered storm reports in one new image show that the Storm Prediction Center received 128 tornado reports on March 2.

To see how the March 2 outbreak compared with the April 27, 2011, outbreak — the largest tornado outbreak in recorded history — meteorologists at the Storm Prediction Center compared the tornado and severe weather warnings issued during those outbreaks (image below). On the March 2 outbreak, the area of warning covered nearly 500,000 square miles (1.3 million square kilometers).

During the April 27 outbreak, the area of warning covered more than 1 million square miles (2.6 million square km).The National Oceanic and Atmospheric Administration created a map of areas with high rotational velocity — an important ingredient for forming tornadoes — using data from NOAA's network of NEXRAD radar installations, processed by the National Severe Storms Laboratory in Norman, Okla. By examining these images, forecasters can determine the approximate tracks of so-called supercells, which feature strong rotation and are known for producing tornadoes. Some of these supercells had rotational velocity up to 180 mph (290 kph), and so their signature stands out from the surrounding storm areas. Features such as these are watched carefully for possible tornado outbreaks.

An from NOAA's Suomi NPP VIIRS instrument shows the overshooting cloud tops and intense storms associated with March 2 tornado outbreak. The imagery was acquired over Illinois, Indiana, Ohio and Kentucky. Polar-orbiting satellites track the subtle changes in the environment that can trigger potentially deadly weather conditions, from tornadoes to tropical storms. Current operational POES data was critical for issuing watches and advisories days in advance of this outbreak.

You can follow OurAmazingPlanet staff writer Brett Israel on Twitter: @btisrael. Follow OurAmazingPlanet for the latest in Earth science and exploration news on Twitter @OAPlanet and on Facebook.

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