Table 1.

Measures used to describe structural complexity and their contribution to calculating composite song complexity scores using principal components analysis (PCA).

measure of complexity	description	PCA
		loadingsa	communalitiesb
song length	period between first and last note of song. Notes are units of continuous sound traces on the spectrogram [12,13]	0.836	0.699
syllable abundance	number of note complexes (syllables) per song [12,13]	0.959	0.919
syllable diversity	number of different syllable types per song [12,13,18–20]	0.952	0.906
transition versatility	number of unique transitions between two particular syllable types per song [12]	0.959	0.919
		eigenvaluec: 3.44 % variance explained: 86.10

^aCorrelation between component and measure values.

^bProportion of measure variance explained by the component.

^cOverall variance of transformed data.