Echolocation calls of free-flying Himalayan swiftlets (Aerodramus brevirostris)

Here, we present our findings of free-flying echolocation calls of Himalayan swiftlets (Aerodramus brevirostris), which were recorded in Shenjing Cave, Hupingshan National Reserve, Shimen County, Hunan Province in June 2012, using Avisoft-UltraSoundGate 116(e). We noted that after foraging at dusk, the Himalayan swiftlets flew fast into the cave without clicks, and then slowed down in dark area in the cave, but with sounds. The echolocation sounds of Himalayan swiftlets are broadband, double noise burst clicks, separated by a short pause. The inter-pulse intervals between double clicks (99.3±3.86 ms)were longer than those within double clicks (6.6±0.42 ms) (P<0.01). With the exception of peak frequency, between 6.2±0.08 kHz and 6.2±0.10 kHz, (P>0.05) and pulse duration 2.9±0.12 ms, 3.2±0.17 ms, (P>0.05) between the first and second, other factors—maximum frequency, minimum frequency, frequency bandwidth, and power—were significantly different between the clicks. The maximum frequency of the first pulse (20.1±1.10 kHz) was higher than that of second (15.4±0.98 kHz) (P<0.01), while the minimum frequency of the first pulse (3.7±0.12 kHz) was lower than that of second (4.0±0.09 kHz) (P<0.05); resulting in the frequency bandwidth of the first pulse (16.5±1.17 kHz) longer than that of second (11.4±1.01 kHz) (P<0.01). The power of the first pulse (?32.5±0.60 dB) was higher than that of second (?35.2±0.94 dB) (P<0.05). More importantly, we found that Himalayan swiftlets emitted echolocation pulses including ultrasonic sound, with a maximum frequency reaching 33.2 kHz.