A group of songbirds may have avoided a devastating storm by fleeing their US breeding grounds after detecting telltale infrasound waves. 
 
 Researchers noticed the behaviour after analysing trackers attached to the birds to study their migration patterns. They believe it is the first documented case of birds making detours to avoid destructive weather systems on the basis of infrasound. 
 
 The golden-winged warblers had just returned from South America to their breeding grounds in the mountains of Tennessee in 2013 when a massive storm was edging closer. Although the birds had just completed a migration of more than 2,500km, they still had the energy to evade the danger. 
 
 The storm, which spawned more than 80 tornadoes across the US and killed 35 people, was 900km away when the birds, apparently acting independently of one another, fled south, with one bird embarking on a 1,500km flight to Cuba before making the return trip once the storm had passed. 
 
 A tornado in Brisco County, Texas. The birds didn’t appear to have used changes in pressure, wind speed or precipitation to warn them of the approaching storm. Photograph: Reed Timmer/Jim Reed Photography/Corbis 
 
 “We looked at barometric pressure, wind speeds on the ground and at low elevations, and the precipitation, but none of these things that typically trigger birds to move had changed,” said David Andersen at the University of Minnesota. 
 
 “What we’re left with is something that allows them to detect a storm from a long distance, and the one thing that seems to be the most obvious is infrasound from tornadoes, which travels through the ground.” 
 
 The scientists had fitted trackers to 20 golden-winged warblers in 2013. Only nine returned to their breeding ground after migrating to South America. Of those nine, the researchers trapped and analysed the flight histories of five. All took evasive action to avoid the storm. 
 
 The birds started to leave their breeding grounds on 27 April 2013, when the storm was whipping up tornadoes in Oklahoma, Kansas and Texas. The next day, with the storm about 100km from their breeding site, the birds had moved a few hundred kilometres south east. When the storm moved over the study area, battering it with winds of up to 160 kilometres per hour, the warblers were on Florida’s Gulf Coast. One flew on to Cuba. 
 
 “In five to six days, they all made this big move around the storm,” Andersen said. “They all went south east in front of the storm, and then let it go by, or moved behind it. It was individual behaviour, they were several hundred kilometres away from each other most of the time.” Details are reported in the journal Current Biology. 
 
 The scientists cannot be sure that the birds picked up infrasound waves from the storm, but previous work in pigeons has suggested that birds might use infrasound to help them navigate. Infrasound waves range from about 0.5Hz to 18Hz, below the audible range of humans. 
 
 The discovery of the evasive action could be good news, said Andersen. “With climate change increasing the frequency and severity of storms, this suggests that birds may have some ability to cope that we hadn’t previously realised. These birds seemed to be capable of making really dramatic movements at short notice, even just after returning on their northwards migration,” he said. 
 
 Had the storm arrived a couple of weeks later, the birds may not have taken flight. By that time, they would have been nesting, and females especially may have been less likely to flee. “It’s hard to say what would happen. It may be more advantageous to survive than stay with a nest that is going to be destroyed anyway,” Andersen said. 
 
 “Biologists had not been looking at the use of infrasound in this way, but it certainly makes sense to me,” said Jon Hagstrum at the US Geological Survey in California, who has studied infrasound use by pigeons. “We may find that acoustics are a pretty significant way that birds in general view their environment, much like dogs use olfaction and humans use sight.” ||||| Birds 'heard tornadoes coming' and fled one day ahead 
 
 Multiple tornadoes devastated parts of the southern and central US in April 
 
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 US scientists say tracking data shows that five golden-winged warblers "evacuated" their nesting site one day before the April 2014 tornado outbreak. 
 
 Geolocators showed the birds left the Appalachians and flew 700km (400 miles) south to the Gulf of Mexico. 
 
 The next day, devastating storms swept across the south and central US. 
 
 Writing in the journal Current Biology, ecologists suggest these birds - and others - may sense such extreme events with their keen low-frequency hearing. 
 
 Remarkably, the warblers had completed their seasonal migration just days earlier, settling down to nest after a 5,000km (3,100 mile) journey from Colombia. 
 
 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 nest and breed in this region every summer, and can be spotted around the Great Lakes and the Appalachian Mountains. 
 
 The golden-winged warblers were being tracked as part of a pilot study of their normal, seasonal migration 
 
 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. 
 
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 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 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. 
 
 Between 27 and 30 April 2014, the huge weather system unleashed at least 84 tornadoes 
 
 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. 
 
 He also cautioned, however, that because the birds' locations were calculated from light readings rather than using satellite tracking, they should be interpreted with caution. 
 
 Changes in the weather or other factors shading the geolocators can interfere with those calculations. 
 
 But the study's authors said the changes in day length recorded by the geolocators were smooth, suggesting they did indeed reflect migration rather than shading errors. 
 
 The sheer size of the calculated movement, from all five birds under observation, also indicates a real "evacuation", Dr Streby said. 
 
 "In cloudy or stormy weather, those estimates can be off by a couple of hundred kilometres. But our worst estimate during any cloudy day was nowhere near as far off as these movements were. 
 
 "I'm confident that they went to the gulf coast." 
 
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– Days after five tiny golden-winged warblers finished their seasonal migration, flying 3,100 miles from Colombia to Tennessee, they vacated their new home to travel 400 miles south to the Gulf of Mexico. Some 24 hours later, on April 27, 2014, at least 84 tornadoes left a path of death and destruction throughout the central and southeastern US, the BBC reports. The birds' flight was no coincidence, say researchers, who suspect the birds' low-frequency hearing allowed them to pick up infrasound waves from a storm that was 560 miles away when they fled; they left before changes in barometric pressure, wind speeds, or precipitation registered, the Guardian reports. By May 2, the birds had all returned. Some 20 warblers were tagged in May 2013 before they flew to Colombia; roughly half returned to the US. Researchers were only able to capture five. "The fact that they came back with the geolocators was supposed to be the great success of this season," says a researcher. "Then this happened!" An expert adds, "It's very unlikely that this species is the only group doing this." And the study indicates birds may be able to make quick and "dramatic" movements to cope with the "predicted increase in severity and frequency of similar storms" brought on by climate change, even after having just completed a long journey, says David Andersen, who was part of the study published in Current Biology. But an ecologist not associated with the study advises that the findings be taken with a grain of salt, as the geolocators gauged location based on light readings rather than satellites. (Find out how birds stopped growing teeth.)