A team of researchers working in France has analysed over 116,000 recordings of birdsong covering 3,160 passerine species, and found that the enormous variety resolves into eight elementary motifs. The work is published in Science, with lead author Quentin Bacquele of Saint-Etienne and Montpellier universities.
The eight are three kinds of trill at different speeds, three kinds of whistle differing in how the pitch moves, and two more complex structures the team calls atonal chaotic notes and harmonic stacks. On average a species draws on about five of the eight, and close to two thirds of songs combine several into something longer. The diversity is real, in other words, but it is combinatorial rather than unlimited.
The geography is the part that turns a catalogue into a finding. Simpler motifs, flat whistles in particular, dominate in tropical environments, where a bird has to be heard through dense vegetation and a crowded soundscape. Complex motifs such as ultrafast trills are more common in temperate regions, where territories are smaller, vegetation is thinner and the breeding season is short, so a song carrying more information in less time is worth more.
That gives the field something it has lacked: a small, shared vocabulary for comparing songs across species and continents, rather than describing each one on its own terms. It also gives conservation monitoring a tractable unit. Recording equipment is cheap and always on, and a system that can count eight motifs can measure what is singing in a habitat, and what has stopped.