Multiple lines of evidence suggest that the latitudinal gradient
in song is caused by sexual selection. Higher latitudes have been
associated with increased sexual selection in birds (Spottiswoode &
Møller 2004; Albrecht et al. 2007), and latitudinal gradients in the
rate of song evolution (Weir & Wheatcroft 2011) and in the elaboration
of sexual signals are often reported (e.g. Bailey 1978; Scott &
Clutton-Brock 1989; Irwin 2000).