Birdsong, in particular, is a sexual signal that varies in multiple
aspects, from the phonology and complexity of individual syllables
to their syntactical organization for building songs (Catchpole &
Slater 2008). Despite its potential for fast evolution (Irwin 1996),
song, or some song traits, is often found to retain high phylogenetic
signal (e.g. Van Buskirk 1997; Päckert et al. 2003), that is, similarity
among species reflects phylogenetic proximity (Revell et al. 2008),
and in some cases song correctly predicts those phylogenetic
relations (Payne 1986; Price & Lanyon 2002, 2004). If sexual
selection leads to complex songs and acts as a diversifying mechanism,
then it could reduce the likelihood of homoplasy and instead