Principle component analysis on seven nightingale song parameters, showing unrotated component loadings.

PC1 represents song output parameters, and PC2 represents structural song parameters. Loadings of variables that made an important contribution to the components are indicated in bold. High scores on PC1 indicate high song rates but short durations of pauses and of song interruptions, and low numbers of interruptions; high scores on PC2 are mainly related to long song lengths and high percentage of songs with trills and initial whistles.