Describing the echolocation calls and behaviour of bats

Too often biologists use terms such as constant frequency (CF) and frequency modulated (FM) to describe the echolocation behaviour of bats or the bats themselves. While CF and FM describe echolocation calls or their components, they do not accurately depict the bats or their echolocation behaviour. By definition, over some period of time a CF signal has no bandwidth (0 kHz), while an FM signal has bandwidth (> 0 kHz). The echolocation calls produced by a wide range of species reveal that aerial feeding bats (those that use echolocation to detect, track and assess airborne prey, usually flying insects) produce high intensity echolocation calls that may include both CF and FM components. Depending upon the frequencies dominating the echolocation calls of these bats, they are detectable by most bat detectors at distances of 5 to > 10 m. In contrast, gleaning bats (those that take prey from surfaces), species that eat blood, and those that visit plants, produce low intensity echolocation calls detectable at distances of 30% of the time). These bats can broadcast and receive at the same time. Although 'CF' accurately describes the components of some echolocation calls, it does not accurately portray the echolocation behaviour or the bats themselves. Variation in the calls of echolocating bats complicates the business of identifying species by these features alone.