Most of the vocalizations produced by bats are search phase calls. These calls are used to detect what is present in the vicinity of a bat, be it food or obstacles that the bat must navigate around. These calls are often species specific and can thus be used to identify the type of bat making the call.

Even within a species, search phase calls typically vary depending on the information that the bat needs. For example, when flying in an area with lots of vegetation, the bat produces more search phase calls, which tend to be higher pitched, and sweep through a greater range of frequencies. More frequent calls provide more rapid information on something that the bat might need to avoid as it flies along. The higher pitch results in a sound with a shorter wavelength and thus provides a finer resolution when an echo is received. Hence, while it is possible to characterize the calls of most species of bats, there is a considerable amount of variation depending on where it is flying and what type of information the bat is trying to obtain.

About the Recordings and Graphs

Each recording is a series of bat calls. With one exception, the pitch (frequency) has been lowered by a factor of 16 so the calls fall within the range of human hearing. For example, a call that was originally 64 kHz is played back at 4 kHz. The Western mastiff bat calls are not lowered since they are already within the range of human hearing. For most of the files, playback speed has not been altered, so you are hearing the calls at the speed they were produced.

The graphs show the pitch (frequency) of the call versus time. Most of the graphs show frequency up to 80 kHz, but the California myotis graph goes up to 120 kHz since the call has a higher pitch. The time scale has been compressed by removing all time between calls. If this were not done, only one or two calls would display on each graph. Hence, the duration of each call on the graph is accurate, but the time interval between calls is not shown. Since the wav files exactly match the corresponding graph, you can get a good feel for time by looking at the graph while playing the call.

• California myotis (Myotis californicus) 600 kb
• Silver-haired bat (Lasionycteris noctivagans) 1.3 mb
• Big-brown bat (Eptesicus fuscus) 503 kb
• Mexican free-tailed bat (Tadarida brasiliensis) 509 kb
• Pallid bat (Antrozous pallidus) 356 kb
• Western mastiff bat (Eumops perotis) 1 mb