Table 1. Description of acoustic variables used in the analysis. Asterisks mark the acoustic variables used to estimate the vocal clusters.

Acoustic feature	Description
Duration [ms] *	Time between onset and offset of call
Amplitude gap [ms] *	Duration of breaks in amplitude within call
PF start [Hz] *	Start frequency of peak frequency
PF mean [Hz]	Mean frequency of peak frequency
PF max [Hz] *	Maximum peak frequency
PF max loc *	Location of PF max in relation to total call duration [(1/duration) * loc]
PF jump [Hz] *	Maximum difference of peak frequency between successive bins
PF modulation [Hz]	Mean frequency differences between original PF and floating average
Slope of trend *	Factor of the linear trend of the peak frequency through the peak frequencies of consecutive 0.21 ms bins
PF sharpness [Hz]	Width of PF peak at 25% amplitude below the maximum amplitude peak