The axis of song variation with the latitudinal
gradient (song PC1) reflects overall song elaboration, namely song
length and complexity of syllables, which are indicative of sexual
selection. Song elaboration to the extent observed in these species
(long songs with many and diverse syllables, cf. Appendix Fig. A1
and Cardoso &Mota 2007) is unnecessary for species or individual
recognition and, on the contrary, may even be detrimental for
those functions by reducing within-individual song stereotypy
(Nelson 1988).