Table 2 Classification of the 15 different syllables types and grouping into 3 different syllable classes used in this study. Ambiguous syllables or other sounds were verified by acoustical inspection

(<  5 ms)

us

ultrashort

Syllables < 91 kHz that are < 5 ms regardless of the shape

[39]

(<  10 ms)

s

short

Syllables < 91 kHz that are < 10 ms regardless of the shape

[26]

f

flat

Syllables < 91 kHz with < 5 kHz frequency modulation

[26]

d

down

Syllables < 91 kHz that decreases in frequency for > 5 kHz

[26]

up

up

Syllables < 91 kHz that increase in frequency for > 5 kHz

[26]

u

u-shaped

Syllables < 91 kHz that first decrease, and then increase in frequency for > 5 kHz each

[26]

ui

u-shaped inverted

Syllables < 91 kHz that first increase, then decrease in frequency for > 5 kHz each

[26]

c

complex

Syllables < 91 kHz that contain ≥2 directional changes in frequency and > 5 kHz modulation of frequency

[26]

c2

complex 2

Syllables < 91 kHz consisting of 2 elements separated by 1 frequency-jump without time separation

[20]

c3

complex 3

Syllables < 91 kHz consisting of 3 elements separated by 2 frequency-jumps without time separation

[20]

c4

complex 4

Syllables < 91 kHz consisting of 4 elements separated by 3 frequency-jumps without time separation

Added category for our classification

c5

complex 5

Syllables < 91 kHz consisting of ≥5 elements separated by ≥4 frequency jumps without time separation

Added category for our classification

h

harmonic

Syllables < 91 kHz that have an harmonic element

[26]

(>  91 kHz)

uh

All syllables > 91 kHz regardless of the shape

[23]

uc

Syllables that do not fit any other of the 14 categories due to background noise or that lack clearly defined spectrographic features (shape)

[40, 41]