The efficacy of Anabat ultrasonic detectors and harp traps for surveying microchiropterans in south-eastern Australia

Insectivorous bats were surveyed using harp traps and ultrasonic bat detect ors at 284 sites in three regions of Victoria, south-eastern Australia. Ten species were recorded in the Box-Ironbark region, 16 species in North-east Victoria, and 14 species in Gippsland. Detectors yielded more species reco rds per site in the Box-Ironbark region (where forests are dry, open and fr agmented), whereas harp traps were more successful in the North-east and Gi ppsland (where forests are generally denser and taller). Results suggest th at detectors are likely to be most effective where there are fewer potentia l species to be considered in call identification, and that harp traps are likely to be most effective in areas of relatively dense vegetation with di screte flyways. Overall the two techniques were complementary. as they were subject to different biases in the suite of species most likely to be reco rded. A library of over 250 ultrasonic reference calls was compiled as the basis for the identification of unknown calls. There was a high level of va riation in the search-phase calls produced by each species, and between 15 and 40 reference calls were considered necessary to gain an adequate repres entation of this variation within the study area. This high level of intra- specific variability, combined with a considerable inter-specific overlap i n call characteristics, resulted in a conservative approach to call identif ication. The development of an identification key was useful in alleviating problems of subjectivity and lack of repeatability associated with qualita tive methods of call identification. Investigation of optimal sampling peri ods revealed that between three and five hours of recording after sunset wa s required to obtain records of approximately 70% of the species identified from a complete night of detector data. Suggestions are provided for the d esign of future surveys using harp traps and detectors.