Source levels of echolocation signals vary in correlation with wingbeat cycle in landing big brown bats (Eptesicus fuscus)

Recordings of the echolocation signals of landing big brown bats with a two-dimensional 16-microphone array revealed that the source level reduction of 7.dB per halving of distance is superimposed by a variation of up to 12.dB within single call groups emitted during the approach. This variation correlates with the wingbeat cycle. The timing of call emission correlates with call group size. First pulses of groups containing many calls are emitted earlier than first calls in groups with fewer calls or single calls. This suggests that the emission of pulse groups follows a fixed motor pattern where the information gained from the preceding pulse group determines how many calls will be emitted in the next group. Single calls and call groups are centred at the middle of the upstroke. Expiration is indicated by call emission. The pause between groups is centred at the middle of the downstroke and indicates inspiration. The hypothesis that the source level variation could be caused by changes in the subglottic pressure due to the contraction of the majorflight muscles is discussed.