Lutra 51(2)_Van De Sijpe_2008

Flight height of trawling pond bats and Daubenton’s bats

This article presents an indirect way to estimate the flight height of trawling bats by measuring the small time intervals between pulses and echoes from the water surface. A particular study site, the historical Vauban ramparts around the town of Ieper, Belgium, allowed for time expansion recordings made from trawling pond bats (Myotis dasycneme) and Daubenton’s bats (M. daubentonii) when they were skimming the water surface close to the wall. The surface echoes were used to estimate the flight height. This method was confirmed by a detailed analysis of a feeding buzz performed by a pond bat close to the wall, where the bat finally broke the water surface, which was revealed by expanding waves in the water. The pulse-echo time interval gradually decreased from search to buzz phase, corresponding with a gradually decreasing flight height from circa 53 to circa 6 cm. Sound analysis of bat passes of Daubenton’s bats and pond bats in the search phase revealed the pond bat to have higher flight heights (median: 43 cm) than Daubenton’s bat (median: 24 cm).