This video from the USA says about itself:
Flight of the Mexican Free-tailed bat at Bracken Cave
16 May 2011
We watched these Mexican Free-tailed bats flying out of Bracken Cave for over an hour. With an estimate of more than 20 million bats, this is the largest colony in the world. It takes around 4 hours for the entire colony to fly out to hunt. In this video, they are leaving to fly approximately 50 miles to feast on boll weevils in Texas cotton fields.
From New Scientist in the USA:
Bats jam each other’s sonar to steal meals
06 November 2014 by Penny Sarchet
Species: Mexican free-tailed bat (Tadarida brasiliensis)
Habitat: Large caves, hollow trees, bridges and attics ranging from Brazil to Nebraska in the US.
It’s frustrating when your smartphone loses its signal in the middle of a call or when downloading a webpage. But for bats, a sudden loss of its sonar signal means missing an insect meal in mid-flight. Now there’s evidence to suggest that bats are sneakily using sonar jamming techniques to make their fellow hunters miss their tasty targets.
Like other bats, the Mexican free-tailed bat uses echolocation to pinpoint prey insects in the dark. But when many bats hunt in the same space, they can interfere with each other’s echoes, making detection more difficult.
Jamming happens when a sound disrupts a bat’s ability to extract location information from the echoes returning from its prey, explains Aaron Corcoran of Johns Hopkins University in Baltimore, Maryland. Previous research has shown that Mexican free-tailed bats can get around this jamming by switching to higher pitches. Using different sound frequencies to map the hunting grounds around them allows many bats to hunt in the same space.
In these studies, jamming of each other’s signals was seemingly inadvertent – a simple consequence of two bats attempting to echolocate in close proximity. But Corcoran has found evidence of sneakier goings-on.
Corcoran has found a second type of sonar jamming in these bats – intentional sabotage of a fellow bat. “In this study, the jamming is on purpose and the jamming signal has been designed by evolution to maximally disrupt the other bat’s echolocation,” he says.
Working with William Conner of Wake Forest University in Winston-Salem, North Carolina, Corcoran compared flight-path calculations with audio and video field recordings. They discovered that these bats emit special ultrasonic signals that interfere with the echolocation of other bats that are attacking insect prey.
“For this type of jamming, the interfering sound needs to overlap the echoes in time and frequency,” says Corcoran. This is unlike previously described accidental jamming, which covers only a single frequency, so can be avoided by shifting to another frequency. “This jamming signal covers all the frequencies used by the other bat, so there’s no available frequency to shift to.”
Playing back recordings of these jamming calls was enough to make flying bats miss their insect targets. Corcoran suggests being jammed by another bat is like having your sense of where an insect is become blurry. “The bats know a moth is there, they just can’t quite know where it is with sufficient detail to capture it.”
He believes the bats do this to each other because they live together in huge numbers, putting them in tough competition for the same food. “This species has the largest aggregations of mammals on the planet – up to one million individuals in a single cave,” explains Corcoran. Leaving a moth for another bat and going to look for another costs more energy. “It’s all about efficiency of getting food. Under high levels of competition, it pays to stay and fight for a food item.”
But it’s a game that one insect has evolved to get in on too. Grote’s bertholdia moth (Bertholdia trigona) is the only moth known to jam bat echolocation to protect itself from being caught and eaten. Corcoran says it jams most bats that have been studied, by making a barrage of high-frequency clicks as a bat approaches them. Far above the range of human hearing, it’s a life-or-death audio battle out there.
Journal reference: Science, DOI: 10.1126/science.1259512
Bertholdia trigona, a moth native to the Arizona desert, emits ultrasonic clicks at a rate of 4,500 times per second to blur bats’ acoustic vision: here.
Bats change their tune to cope with human noise pollution: here.
To find ways to improve human-made active sensing, scientists worldwide study the sonar systems of bats and dolphins: here.