In many songbird species, male song serves as an important cue in female mate choice [1,2] and may be used to defend territories from invasion by conspecific males [3]. (rsos.royalsocietypublishing.org)
[Christopher D. Rodeheffer, Randi P. Proffitt Leyva and Sarah E. Hill, Attractive Female Romantic Partners Provide a Proxy for Unobservable Male Qualities: The When and Why Behind Human Female Mate Choice Copying] (scientificamerican.com)
Variation in song not only aids in species recognition, but is particularly useful for female mate choice because it allows females to discriminate between males from local versus non-local populations. (rsos.royalsocietypublishing.org)