LOS ANGELES - 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 the University of California, 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 skeptical 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 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 have left their coveted mating grounds after such an arduous migration. Then they remembered how, in late April, ferocious storms sent them fleeing for shelter.
Previous research has shown that birds can sense infrasound. 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, Streby said.