Picking the "right" bat detector - time expansion versus zero-crossing.

M. Brock Fenton, CBCB, Royal Ontario Museum, Toronto, Ontario, Canada M5S 2C6

Direct comparisons revealed that a bat detecting system using zero-crossing period meter analysis (the Anabat II Bat Detector with Anabat ZCAIM module and Anabat 6 software) was significantly less sensitive to bat echolocation calls than time-expansion bat detecting systems. One time expansion system involved recording the high frequency output of a QMC S200 Detector to a Racal Instrumentation tape recorder operated at 30 inches per second, with analysis of slowed down recordings by Canary¨. The other time expansion system was a Pettersson D980 bat detector with digitally time expanded output recorded on a SONY DCD 100 dat recorder and analysed by Bat Sound Pro¨. In a variety of field settings, the time-expansion systems always recorded significantly more echolocation calls than the zero-crossing period meter system. Furthermore, under laboratory and field conditions, the zero-crossing period meter analysis presented significantly different pictures of frequency and time features of echolocation calls, including highest and lowest frequencies in the calls (kHz) and call durations (ms). The differences in performance between the two kinds of systems reflect the fact that to pure tone frequencies of 20, 30, 40 and 60 kHz, the threshold for signal detection by the Anabat system was 19 dB (SPL A @ 4 cm) higher than for either time expansion system. These results mean that zero-crossing systems should not be used to characterize the echolocation calls of bats and that they are of limited value in studies of distribution and habitat use.