Detection and avoidance of harp traps by echolocating bats

Researchers often use harp traps and mist nets to capture bats, and need to be aware of factors that affect trap capture efficiency. Ultrasound reflects from small targets in a frequency-dependent manner, so we predicted that higher frequency sound pulses Would return stronger echoes from the fine wires and net of bat traps than would lower frequency signals. We also predicted that mist nets would return stronger echoes than harp traps because mist nets are made of a higher density (and often diameter) of material. Ensonification experiments with pulses of 20-110 kHz showed that both harp traps and mist nets reflected higher frequency pulses more strongly. Pocketed areas of mist nets returned stronger echoes than harp traps although at some frequencies differences between trap types were small. Our results provide one verified reason why harp traps are more effective than mist nets at capturing bats, and also predict that bats using high frequency echolocation calls may be more difficult to trap than species emitting low frequency signals. Interspecies differences in how traps are detected are therefore likely sources of bias in field surveys. Observations of bats encountering harp traps in the field showed less than 4% of encounters resulted in capture, and only 8.8% of encounters could be interpreted its a failure to detect the trap. A comparison between two species that differ in echolocation call and flight characteristics (Rhinolophus hipposideros and Myotis nattereri) showed no difference in trap detection or avoidance. However, differences in behaviour during trap encounters were apparent.