A method for determining relative activity of free flying bats using a new activity index for acoustic monitoring

A new activity index for acoustical bat data is presented. The AI (acoustic activity index) was highly correlated to bat passes but proved to be a less biased index of activity. The method dispenses with the need to define, identify and count bat passes and provides a simple means to quantify activity. It uses the Anabat system where acoustic surveys are carried out in real time with the data saved directly to a computer hard drive, taking advantage of the date-time information encoded into each file. The method is based upon the presence/absence of a species occurrence during one-minute time intervals and avoids skewing an index of activity that may reflect the behavior of the species sampled. Examples are given showing that the AI is an effective measure of bat activity allowing comparisons between sites, times and species.