Study reveals the building blocks of birdsong

Key points

• The finding comes from an international team led by researchers from the University of Saint-Etienne and the Ecole Pratique des…
• Birdsong can seem almost limitless: one species whistles, another rattles, another races through a rapid sequence of notes.
• They were looking at the architecture of birdsong — how sounds change in pitch and speed over time, and how…
• What emerged was an acoustic toolkit of eight motifs: slow, fast and ultrafast trills; flat, slow-modulated and fast-modulated whistles; harmonic…

Birdsong can seem almost limitless: one species whistles, another rattles, another races through a rapid sequence of notes. Yet beneath this extraordinary variety lies a shared musical vocabulary. Across more than 3,000 songbird species, scientists have identified eight basic acoustic motifs that birds mix and match to create their songs.

The finding comes from an international team led by researchers from the University of Saint-Etienne and the Ecole Pratique des Hautes Etudes–PSL in France. In a study published in Science on July 23, researchers analysed more than 1,16,000 recordings from 3,160 passerine species, one of the largest global examinations of birdsong to date.

The researchers were not simply counting chirps. They were looking at the architecture of birdsong — how sounds change in pitch and speed over time, and how different elements are combined.

What emerged was an acoustic toolkit of eight motifs: slow, fast and ultrafast trills; flat, slow-modulated and fast-modulated whistles; harmonic stacks; and chaotic notes. A bird’s song can then be thought of as a combination of these basic elements, rather like a musical composition assembled from a limited set of notes.

What happened

Yet the same eight building blocks do not produce the same music everywhere.

Biology and habitat influence the mix. The researchers found that the acoustic choices produced by birds are linked to their biology, behaviour and the environments in which they live. While nearly two-thirds of songs contain multiple motifs, on average, a passerine species uses around five of the eight motifs.

Consider a bird living deep inside a tropical rainforest. Dense vegetation can interfere with the transmission of sound, making complicated signals harder to send over long distances. 

In these environments, birds are more likely to apply simpler motifs, particularly flat whistles and slow trills. It are more resistant to degradation as they travel through vegetation.

The details

The pattern appears remarkably consistent across tropical regions. Similar acoustic strategies have been found in rainforests separated by thousands of kilometres, suggesting that unrelated birds can arrive at similar solutions when confronted with similar physical difficulties.

Temperate regions tell a different story. There, birds are more likely to apply complex, information-rich motifs such as ultrafast trills. These sounds can pack considerable information into a short period, but they are also more vulnerable to distortion over distance. Why favour them? While shorter breeding seasons and higher population densities can intensify competition for mates, the researchers suggest that communication in these environments is more often at close range. In such circumstances, elaborate songs may offer greater advantages.

It is essentially a trade-off: simplicity travels better; complexity can communicate more.

But geography is only part of the story. The bird itself matters, too. The researchers found associations between birdsong and biological traits including social organisation, morphology and mating system. Body and beak size, for example, can influence what sounds a bird is physically capable of producing. While those communicating at closer range can make greater apply of faster and more complex sounds, species that communicate over longer distances tend towards simpler motifs.

Why it matters

Mating behaviour also enters the picture. More complex motifs were associated with species in which sexual competition is stronger, including species where males mate with multiple females. In such settings, a more elaborate vocal performance may help birds compete for mates or signal information concerning the singer.

That global scale is important. Birdsong has fascinated scientists for decades, but comparing songs between species and continents is difficult when every species appears to have its own musical language. The eight-motif framework provides a common vocabulary with which those songs can be compared.

It also changes how a familiar morning chorus might be heard. While the Bohemian Waxwing produces an ultrafast trill, a Northern Cardinal, for instance, is associated with slow trills. While the American Crow can produce harmonic stacks, the Eastern Wood-Pewee uses a slow-modulated whistle. These birds may sound completely different to the human ear, yet their songs can be described using elements from the same acoustic toolkit.

And that toolkit may reveal more than simply what a bird sounds like.  If birdsong is partly shaped by the physical environment, then changes to that environment could affect communication itself. 

Background

Habitat destruction, increasing background noise and changes in vegetation could alter the acoustic conditions in which birds evolved to communicate. Understanding the relationship between habitat and song could therefore help scientists examine how birds respond when their acoustic landscapes change.

For lead researcher Quentin BacquelE, the work also offers a new way to think concerning birdsong: not as an endless collection of unrelated melodies, but as variations on a surprisingly small set of acoustic ideas.

And the framework may not stop with birds. The researchers suggest that a similar approach could eventually be applied to other forms of animal communication, from frogs and insects to mammals.