In 1938, a young student from Harvard University solved a mystery that was first described in the 18th century, when zoologists discovered that bats with experimentally blocked ears crash into objects, whereas blinded bats can navigate just fine. The student, Donald Griffin, found out what is general knowledge today: bats echolocate to navigate, i.e. they send out sound waves and use the echo that is produced when the sound waves hit an object to find obstacles in their flight path. But it appears that we still do not know everything about bat navigation. A recent study suggests that bats ‘whisper’ or even completely switch off echolocation under certain circumstances and fly ‘blind’, which could explain the high percentage of bats killed by wind turbines.
Aaron Corcoran from the Wake Forest University, USA, and Theodore Weller from the USDA Forest Service investigated the use of echolocation in hoary bats, the species that is killed most frequently by wind turbines in the USA. The researchers performed a series of three experiments recording bat calls and their flight paths: (1) under natural conditions, (2) in response to an introduced obstacle and (3) in response to echolocation playbacks. Previous field recordings had suggested that bats might not always use echolocation, but these observations had been challenged because it was not clear whether the bats were always in the range of the microphones. Therefore, the researchers of the current study used a combination of time-synchronised ultrasound microphones and infrared cameras. The cameras captured the exact flight path of the bats and, together with the known position and the recording direction of the microphones, they were able to determine the exact location that each recorded call was made from.
Surprisingly, Corcoran and Weller only recorded examples of previously known hoary bat echolocation calls in 8% of the recorded flights and the bats flew about half of the time without any recordable echolocation. Furthermore, the scientists detected a previously unknown call type that the bats used in the remaining 43% of recorded flights. The call, which they termed a ‘micro’ call, was shorter and had a much lower sound pressure level (volume) than known echolocation calls. Importantly, the low pressure level drastically reduces the detection range, making the call ineffective for flight navigation at high speed or insect detection. And, in the cases in which the bats flew into the mist-net that the researchers deployed as an obstacle, they were either using the micro calls or no calls at all when approaching the net and echolocation was only used shortly before the collision occurred – too late to avoid being caught.
As the study took place during the hoary bat mating season and the researchers recorded the micro calls when the bats were approaching members of their own species or when flying towards a speaker that was playing bat calls, the researchers concluded that bats might produce micro calls to avoid other bats eavesdropping on their conversations during the mating season. Although micro calls would still allow bats to detect trees from a distance of 7.5 m, which is early enough to avoid a collision, the situation might be different, unfortunately, when it comes to rotor blades moving at high speed, and might partly explain the high number of hoary bats killed by wind turbines in the autumn breeding season.