Attracting bats using sound is a specialized technique used to actively draw animals toward a specific location for purposes like population monitoring, research, or natural pest control. This method relies on broadcasting recorded sounds to elicit a behavioral response from the bats, encouraging them to approach the source. While most people are familiar with echolocation, the sonar system bats use for navigation and hunting, acoustic attraction targets a different category of their vocalizations. The success of this approach depends on understanding the subtle differences between these two types of sound production.
Bat Social Calls: The Basis for Acoustic Attraction
The foundation of acoustic attraction lies in the difference between echolocation calls and social calls. Echolocation is a series of ultrasonic pulses used primarily for spatial orientation, allowing a bat to build a mental map of its surroundings and locate prey. These calls are typically high-frequency and function over short distances, optimizing detail and resolution.
In contrast, social calls are the primary means of communication between bats, functioning much like language for information transfer. These vocalizations serve various purposes, including territorial defense, coordinating group movements, or advertising for mating partners. For many bat species, social calls are often lower in frequency than echolocation pulses, a feature that allows the sound to travel farther and attenuate less rapidly.
Acoustic attraction specifically utilizes these communication signals, aiming to mimic the presence of conspecifics or a suitable social gathering. Broadcasting a contact call can signal to a passing bat that a roosting spot is occupied, suggesting a safe or valuable location.
Methods and Technology of Acoustic Luring
The practical application of acoustic luring involves specialized equipment designed to generate and broadcast specific sound patterns. Devices known as acoustic lures or ultrasonic playback systems are used to emit synthesized or recorded bat calls. Since the sounds are often ultrasonic—above the 20 kilohertz (kHz) range audible to humans—these systems require high-frequency transducers, or speakers, capable of accurately reproducing the calls.
The effectiveness of an acoustic lure depends on the quality and relevance of the broadcasted call. Researchers often use calls specific to the local bat species, as bats are more attracted to the social calls of their own kind, known as conspecifics. Using a call from a non-native species or one that signals an aggressive interaction may result in no attraction or a repellent effect.
Some studies have successfully used a variety of signals, including social calls, distress calls, and “feeding buzzes,” which are the rapid series of echolocation pulses a bat makes just before capturing prey. A significant technical limitation of acoustic luring is the rapid attenuation of sound, particularly the high-frequency ultrasonic calls, in the air.
This means the attractive effect of the lure is localized, only drawing in bats already in the immediate vicinity of the device. Success requires precise calibration to ensure the frequency and intensity match the natural calls of the target species.
Why Habitat and Environment Override Sound
While acoustic lures can temporarily draw a bat’s attention, they cannot sustain a long-term presence because they fail to satisfy the animal’s fundamental survival needs. A bat may approach the source of a social call out of curiosity or a social drive, but it will not remain without a suitable place to live and a reliable food supply. The physical habitat and local environment ultimately override the effect of a temporary sound stimulus.
For a bat to establish permanent residency, it requires a safe and secure roosting site, such as a bat house or an undisturbed tree cavity, to shelter from predators and the elements. These structures must be correctly positioned, typically 10 to 20 feet high in a location that receives several hours of direct sunlight, which is necessary for maintaining the warm temperatures bats prefer.
Bats are loyal to established roosts, and convincing them to move to a new location can take a long time, sometimes up to two years, even with a perfect habitat setup. Furthermore, the area must provide a plentiful and consistent food source, which primarily means a high density of night-flying insects.
This is supported by the presence of native plants, which attract the insects bats feed on, and a nearby water source for drinking and foraging. Acoustic lures are therefore best viewed as a supplement to a comprehensive habitat strategy, where they might be used to alert passing bats to the presence of a newly installed, high-quality roosting site.