A group of songbirds may have avoided a devastating storm by fleeing their US breeding grounds after detecting telltale infrasound waves, according to new research.
Scientists 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.
"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
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 expecially 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."
Guardian