19 December 2014 Last updated at 02:21

Birds 'heard tornadoes coming' and fled the day before

By Jonathan Webb Science reporter, BBC News

US scientists say tracking data showing five golden-winged warblers left their nesting site a day a tornado outbreak suggests the birds "heard it coming".

Geolocators showed the birds, which had just finished a 5,000km migration, left the Appalachians and flew 700km (400 miles) south to the Gulf of Mexico.

The next day the deadly April 2014 tornadoes swept across the central US.

The discovery, reported in Current Biology, suggests many birds may sense and escape extreme events in this way.

Dr Henry Streby, from the University of California, Berkeley, said he initially set out to see if tracking the warblers was even possible.

"This was just a pilot season for a larger study that we're about to start," Dr Streby told the BBC.

"These are very tiny songbirds - they weigh about nine grams.

"The fact that they came back with the geolocators was supposed to be the great success of this season. Then this happened!"

Everybody out

Working with colleagues from the Universities of Tennessee and Minnesota, Dr Streby tagged 20 golden-winged warblers in May 2013, in the Cumberland Mountains of north-eastern Tennessee.

The birds are often seen around the Great Lakes and the Appalachian Mountains, where they nest and breed every summer.

After disappearing to Colombia for the winter, 10 of the tagged warblers returned in April 2014. The team was in the field observing them when they received advance warning of the tornadoes.

"We evacuated ourselves to the waffle house in Caryville, Tennessee, for the one day that the storm was really bad," Dr Streby said.

Elsewhere in the US the storm had more drastic consequences. At least 84 tornadoes caused 35 fatalities and more than $1bn (£0.6bn) in property damage.

After the storm had blown over, the team recaptured five of the warblers and removed the geolocators.

These are tiny devices weighing about half a gram, which measure light levels. Based on the timing and length of the days they record, these gadgets allow scientists to calculate and track the approximate location of migratory birds.

In this case, all five indicated that the birds had taken unprecedented evasive action, beginning one to two days ahead of the storm's arrival.

"The warblers in our study flew at least 1,500km (932 miles) in total," Dr Streby said.

They escaped just south of the tornadoes' path - and then went straight home again. By 2 May, all five were back in their nesting area.

Remarkably, the warblers' evacuation commenced while the closest tornado was still hundreds of miles away. Weather conditions in the nesting area were still nothing out of the ordinary.

Distant rumble

The most likely tip-off was the deep rumble that tornadoes produce, well below what humans can hear.

Noise in this "infrasound" range travels thousands of kilometres, and may serve as something of an early warning system for animals that can pick it up.

"It's very unlikely that this species is the only group doing this," Dr Streby said.

Even from casual birdwatching in the area as the storm drew nearer, he said, "It seemed like there were far fewer birds - so I suspect it's not a species-specific trait."

Dr Chris Hewson, a senior research ecologist at the British Trust for Ornithology, told BBC News that infrasound was a very plausible explanation.

He pointed out that several birds, including falcons, are thought to use infrasound to help them navigate.

"And you can see from the weather data that there doesn't appear to be any alternative cue that they could be picking up on," he said.

The new study is the first time that migratory birds have been seen taking such dramatic evasive action.

"We know that birds can alter their route to avoid things during regular migration," Dr Streby explained. "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."

Dr Hewson agreed the finding was "an eye opener".

"It's the sort of thing that we're going to see more of in the future as well, as tracking devices get more miniaturised," he said.

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