elsewhere (see Fenton 1988, and Thomas and West 1989). One or 

 two tunable heterodyne detectors were used during this study. 

 These detectors can be tuned to a number of frequencies, but can 

 only scan a narrow band at one time. Detectors were normally set 

 at 40 kHz during surveys, as most bats in Montana can be detected 

 at that frequency. If a bat was heard long enough, an attempt 

 was made to determine its lowest detectable frequency, as several 

 species, or groups of species, can be identified using this 

 characteristic. When two detectors were used on one survey, one 

 was set at 40 kHz and one at 28 kHz. The lowest frequencies 

 emitted by Myotis species (except M. volans, down to 32 kHz) are 

 around 36 to 38 kHz, thus when a bat was heard on both detectors, 

 it was presumed that it was not a Myotis. If it was only heard 

 on the 40 Khz detector, it probably was a Myotis. 



The intensity of the echolocation call differs between species, 

 as well as the frequency range of the call. This characteristic 

 biases relative density information. Bats with intense 

 vocalizations, such as Hoary bats or Big brown bats , are much 

 more likely to be detected than those with weaker vocalizations, 

 such as Townsend ' s big-eared bat. Myotis species fall between 

 these extremes in intensity of their vocalizations. In effect, 

 the area sampled by the detector is much larger for the strong 

 emitters than for the moderate or weak emitters. Thus, direct 

 comparisons of relative density between species based solely on 

 bat detector results is unwise. 



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