From musical whistles of a red-whiskered bulbul to mechanical whirr of a barn swallow, there is incredible diversity to avian music. But according to a new study by researchers from France and U.S., this vast diversity comes from just eight building blocks.
Researchers analysed more than one lakh song recordings from more than 3,000 passerine bird species worldwide using machine learning to reveal eight core motifs, or basic sounds.
These motifs consist of three types of trills (repetition of same note) distinguished by pitch, three types of whistles based on pitch modulation, and two other complex sounds.
"If you go to different countries in the world, you will hear totally different bird songs," said Quentin Bacqué, one of paper's authors and PhD student at University of Montpellier and Saint-Etienne, France. Central question was whether it was possible to characterise songs into broad types, Bacqué added. Researchers also did not just decompose birdsong down to their sound; they also found where birds reside influences sounds they use.
By sourcing data from Xeno-canto, a citizen-science repository of verified wildlife audio recordings, researchers covered bird songs from almost everywhere on planet. Bacqué said it also meant there was data disparity, with more recordings coming from Europe and Americas than from Africa.
Team de-noised recordings to focus on species of interest, then computed their acoustic signals via spectrograms - visual maps that show sound frequencies over time. Following this, they computed modulation power spectrum for each song, which reduced acoustic signals to constituent patterns of frequency and temporal modulation.
For this study, birds, on average, use five acoustic space, each captured by a specific motif. Motifs included three types of trills - slow, fast, and ultrafast; three types of whistles - flat, slow-modulated and fast-modulated; atonal chaotic noises, which are harsh, rough sounds; and harmonic stacks, where there are several low- and high-pitched tones at once, like in song of a crow.
Main finding is that these motifs serve as building blocks, with bird species using just one or all eight in their songs, said Pooja Choksi, co-founder and lead scientist at India Ecoacoustics Network. She was not involved in study. "This is a new way of looking at diversity and generalise across such a large group of species," Ms. Choksi said. "At the end of day, motifs are simply characteristics of a song, and there will probably always be a debate on how to characterise something."
An MPS can provide broad perspective on timbre, pitch, tonality, and rhythm, Bacqué said. "That's really useful because when you have thousands of songs, you don't want to know every detail. Instead, you want to know how they relate to one another."
Researchers were then able to explore variability of these songs inside a 37-dimensional acoustic space, a 3D representation available online. "It's like all these differently shaped clouds altogether, where proximity reflects spectro-temporal similarities," Bacqué said. With help of machine learning, determining factors: Researchers found that birds communicating over long distances tend to use simple motifs like flat whistles and slow trills whereas those signalling at close range favour faster, more complex motifs. Similarly, simpler sounds are represented better in larger birds and more challenging motifs are predominant in smaller species. Finally, analysis revealed that groups facing more local selection pressures tend to use complex motifs while species subject to weaker pressures use simpler ones.
"Song motifs are essentially a trade-off between encoding more information and transmitting it over long distances," Bacqué said. "And what we found is this trade-off between temperate and tropical forests."
Upon mapping global geographic distribution of motifs, they found that in tropical rainforests such as in Southeast Asia, birds prefer simpler songs that travel efficiently through dense vegetation, but in temperate regions such as Europe and North America, birds inhabit smaller territories and have short breeding seasons, information-rich songs. This is biggest finding: it reveals how environmental constraints shape birdsong at planetary scale. "It makes logical sense that species have adapted their songs for optimal transmission in their local environment," Ms Choksi said. She also questioned whether patterns would hold at much finer spatial resolutions.
"The are broad categories, and within each, there can be a lot of variation in frequency and shape," said Elodie Briefer, associate professor at University of Copenhagen who was not involved. So there is variation among species that share spaces, according to Ms Briefer, whose research focuses on animal cognition and communication.[motifs]
Birds themselves perceive many more nuances within these eight motifs, Ms Choksi said. But it is interesting that birds share so many skylarks. She was, however, interested in building on this research and exploring how birds perceive these eight motifs and whether any factors influence order in which motifs are used.
Ms Choksi said she believes it is worth studying how habitat changes affect birdsong and mating success and reproduction. Particularly true in India, where invasive species have proliferated and tree-planting is almost out of control, she added.
The GIST: Despite immense diversity of birdsong across different regions, study found these sounds can be grouped into limited set of fundamental categories. On average, birds use five of eight song motifs, study says, with selection largely shaped by evolutionary history. Study's most important finding is that environmental constraints influence birdsong patterns, shaping how birds communicate at planetary scale.