Kroodsma emphasizes that we know little about why one or another bird has a specific repertoire. Yet despite the dazzling variety, it appears to me that all birdsongs have general requirements and constraints, and I believe that these shared characteristics may in themselves shed some light on the enigma. The primary requirement of a species' display song is that it must stand out from environmental noise--that is, it must carry--and it must be distinct from competing voices on the stage. Once females reward a specific song type with mating, then success breeds success, and whatever it is that attracts, the male that has more of it enjoys a huge advantage.
But singing is not cheap: the performers are conspicuous to predators, and the displays are so costly in time and energy that the performers may appear to handicap themselves. I doubt, however, that it is the flaunting of handicap as such that attracts the females ("I am so strong and healthy that I have energy to waste on singing"). The singer must cater to the females' taste. As in our own fashions of clothing and music, there is not necessarily rhyme or reason in the specifically chosen attribute, except the most important one--it works.
Konrad Lorenz reputedly said that birdsong is "more beautiful than necessary." It seems to me that it is just as likely that the flamboyant displays of song and dance, of feathers and, in the bowerbirds, of decorated love shacks are indeed necessary, because females compare, and they are picky. Arbitrary though their criteria of choice may be, it is significant that we humans also find many of the same displays beautiful.
This article was originally published with the title How to Listen to Birds.