A method for determining relative activity of free flying bats using a newactivity 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 les s biased index of activity. The method dispenses with the need to define, i dentify and count bat passes and provides a simple means to quantify activi ty. It uses the Anabat system where acoustic surveys are carried out in rea l time with the data saved directly to a computer hard drive, taking advant age of the date-time information encoded into each file. The method is base d upon the presence/absence of a species occurrence during one-minute time intervals and avoids skewing an index of activity that may reflect the beha vior of the species sampled. Examples are given showing that the Al is an e ffective measure of bat activity allowing comparisons between sites, times and species.