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 Scientists said on Thursday that little songbirds known as
golden-winged warblers fled their nesting grounds in Tennessee up to
two days before the arrival of a fierce storm system that unleashed
84 tornadoes in southern U.S. states in April. The researchers said
the birds were apparently alerted to the danger by sounds at
frequencies below the range of human hearing.
The storm killed 35 people, wrecked many homes, toppled trees and
tossed vehicles around like toys, but the warblers were already long
gone, flying up to 930 miles (1,500 km) to avoid the storm and
reaching points as far away as Florida and Cuba, the researchers
said.
Local weather conditions were normal when the birds took flight from
their breeding ground in the Cumberland Mountains of eastern
Tennessee, with no significant changes in factors like barometric
pressure, temperature or wind speeds. And the storm, already
spawning tornadoes, was still hundreds of miles away.
 "This suggests that these birds can detect severe weather at great
distances," said wildlife biologist David Andersen of the U.S.
Geological Survey and the University of Minnesota, one of the
researchers in the study published in the journal Current Biology.
"We hypothesize that the birds were detecting infrasound from
tornadoes that were already occurring when the storm was still quite
distant from our study site," Andersen added.
Infrasound is below the normal limits of human hearing, but some
animals can hear it.
The warblers came right back home after the storm passed, said
fellow researcher Henry Streby, an ecologist from the University of
California, Berkeley.
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The researchers, who were already studying the migratory patterns of
the warblers, tracked their evacuation using transmitters that had
been placed on a small number of the birds.
Golden-winged warblers boast gray plumage marked by patches of
yellow on the head and wings. They weigh about 0.30 ounces (9 grams)
and have a wingspan of about 7.5 inches (19 cm).
The warblers spend winters in Central America and northern South
America before migrating back to the Appalachian Mountain region of
the southern United States and the Great Lakes region of the United
States and Canada to breed.
(Writing by Will Dunham; Editing by Lisa Von Ahn)
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