Further analysis of spectral traits showed that excess attenuation values were mainly affected by minimum frequency and peak frequency, while blur ratio variation was predominantly affected by bandwidth, and signal-to-noise ratios were predicted by bandwidth and peak frequency.
In addition, the selected AUD calls had duration, DF, maximum frequency, minimum frequency and bandwidth within ±1 SD of the mean values of all the audible calls.
The majority of variation in the five variables that measured frequency (start frequency, end frequency, maximum frequency, minimum frequency, and frequency at maximum amplitude; see Figure 1c) was explained by PC1 (Table S1).
Differentiating between vestpertilionid species was particularly difficult; we relied primarily on the minimum frequency and pulse duration [41] to assign species identifications.
Identifications were made by inspecting individual pulses and pulse sequences for key features, primarily pulse shape and bandwidth, frequency of maximum energy (characteristic frequency for Anabat calls), terminal frequency, minimum frequency, and pulse duration.
The five variables that measured frequency (start frequency, end frequency, maximum frequency, minimum frequency, and frequency at maximum amplitude; see Figure 1c) were subjected to a Principal Component analysis to yield a single principal component frequency axis (PC1; details in Table S1).
We found evidence for association between group size and the mean minimum frequency, as well as between mean minimum frequency and duration.
The following acoustic parameters were measured from each call: duration, repetition rate, maximum frequency, minimum frequency, peak spectral frequency, total bandwidth, maximum and minimum frequency (and bandwidth) of the fundamental FM element, and number of harmonics.
Bandwidth (Hz) was defined as the maximum minus the minimum frequency, and was calculated for both the entire call and the fundamental FM element.
It should be noted that May-Collado et al. (2007) [ 59] found a correlation between minimum frequency and body size across whales.
