Washington: Ornithologist Henry Streby was happy enough that a song bird weighing less than two nickels managed to carry a tiny electronic sensor from Tennessee to Colombia and back.
Then he looked at the paths taken by several of the birds.
The data showed that five of his recently returned golden-winged warblers fled their Appalachian Mountain breeding ground and winged back to the Gulf of Mexico a day or two ahead of a massive thunderstorm cell that would later spawn 84 tornadoes and kill at least 35 people.
Streby, a National Science Foundation visiting research scholar at UC Berkeley, thinks the birds may have been reacting to very low-frequency sound waves produced by the distant, approaching storm, according to a study published online Thursday in the journal Current Biology.
“Everybody knows that birds can respond to changes in barometric pressure, wind speed and wind direction, and cloud cover — all the things that come with the front of a storm,” said Streby. “But these birds left long before any of those things happened.”
First, Streby was sceptical that the birds had even left. Maybe the batteries malfunctioned on the tiny sensors, which record only sunlight and time, which then is extrapolated to determine their global position.
“We’re not quite to the point of handing the birds cellphones and getting pinpoint GPS on these tiny, tiny songbirds,” Streby said. “The purpose of our study was just to see if birds this small could carry these things, so we could start tracking their migration.”
It defied odds that all five sensors could have spit out false daylight and timing information in five different ways, Streby said. Each bird’s data suggested they took separate paths — some flew east, some flew west, but four eventually hooked south to the Florida panhandle, while one traced the east coast of Florida and continued to Cuba.
Obligate migrators, which fly long distances seasonally, are known to detour around storms, the study notes. But combining a full round-trip migration with a separate flight sparked by environmental factors (facultative migration) has not been observed among such species, according to the study.
Streby and his colleagues from the universities of Tennessee and Minnesota were stumped about why the warblers would’ve left their coveted mating grounds after such an arduous migration. Then he and his graduate assistants remembered how, back in late April, ferocious thunderstorms sent them fleeing for shelter in a Waffle House in Caryville, Tenn.
Still, weather and sensor data showed the birds would have left 24 to 48 hours before the storms arrived. That means the supercell storm was between 250 miles and 600 miles away, the researchers found. At that distance, none of the wind, barometric pressure or other changes would have been evident, they calculated.
Previous research, however, has shown that birds can sense infrasound — waves at frequencies well below the human audible range and at such long wavelengths that they don’t attenuate, and can propagate for thousands of miles. (The military has studied them to detect distant weapons’ explosions.)
Birds also can detect shifts in sound waves caused by the motion of their source, known as the Doppler effect.
That put the puzzle together plausibly enough to publish, said Streby.
“It’s possible something that hasn’t been discovered explains it, but this really makes sense,” he said. “We know for sure that it makes the sound, we know for sure that they hear the sound, and we know for sure that it travels far enough. And we also know they can sense Doppler changes.”
Biologist John McCormack, director-curator of the Moore Laboratory of Zoology at Occidental College, who was not involved in the study, said he found its hypothesis plausible.
“It’s something that just wasn’t possible to observe before geolocators,” McCormack said.
Streby is hoping other researchers might re-examine data that they may have discarded as anomalous. Conducting a field experiment to test his hypothesis, he said, would be next to impossible. But nature might help.
“It’s not that I’m hoping that next year or the year after there will be a giant storm that pushes through our study,” Streby said. “But at the same time, if that storm does happen, it would be nice to see if this migration does or doesn’t happen again.”