The complex world of birdsongs and mimicry

Some years ago, I came across a fascinating study by BBC on birdsongs that showed the human ear could only detect a subset of the frequencies - by shifting the audio waveform to a human audible frequency it was clear that we were listening to just the tip of the proverbial iceberg. That got me thinking - how complex is the world of birdsongs?

Birdsongs are traditionally defined as the long, often complex learned vocalizations produced mostly by males but also by females to attract mates (typically only during mating season) and defend their territories (year-round). In contrast, bird calls are usually shorter, simpler, innately known (not learned) sounds for more discrete functions, such as signalling about predators and food. 

Analysis of Parus minor, aka Oriental tit, bird calls by Toshitaka Suzuki and his colleagues at The Graduate University for Advanced Studies in Japan show that the shorter coded message involves syntax, or the precise arrangement of chirps, similar to human speech to indicate a precise meaning. One sequence for example refers to predatory snakes, while another to the danger of hawks overhead. Parent birds also have different calls for their chicks, telling them to flee or hide in the face of danger. What is special about Oriental tits is that they seem able to combine at least two of these calls much like a complex sentence.

Audio analysis of birdsongs of Taeniopygia guttata, aka Zebra finches of Australia which are highly social songbirds that live in large groups and maintain life-long pair bonds, shows it is made up of distinctive and relatively simple motifs, consisting of three to eight syllables, with characteristic rhythms and tempos, repeated over and over again, usually in the same order. 

It must be noted that Zebra finches are closed-ended learners, that have a single sensitive period for song learning, after which new songs cannot be learned. Male finches learn motifs, a combination of both the syllables and the sequence in which they occur, from a tutor, typically their father - an evolutionary trait called vocal learning that humans share only with songbirds; other primates including our closest living relatives, chimpanzees, do not appear to be vocal learners. 

While human ears are tuned to variations in pitch or loudness, birds, on the other hand, use the fine structures of the song, imperceptible to our ears, to mine a rich trove of information about emotion, health, age, individual identity, and much more

Evolution has influenced how human speech differs from birdsong

 
Humans produce the sounds for speech with our mouths and tongues using only the larynx, a single source at the top of our neck. Therefore, melodies and sentence structure are essential to us when we listen to music and speech and any changes to the fine structures of the sound disrupt the message and melody. For example, if we change the pitch of the song, we find someone who isn't singing on the right pitch to be rather annoying and discordant, not so with birds.

Birds, in contrast, produce sound using a unique two-branched structure that sits atop the lungs called the syrinx. It carries two sources of sound, one from each branch, that can be controlled independently. On top of that, muscles in the songbird syrinx contract faster than any other vertebrate muscle, enabling millisecond-level temporal control. Thus, birds seem to be listening most closely to fine structures or the acoustic details of individual song elements, independent of the sequence in which they occur. And they hear details beyond what our ears and brains can discern.

Recent studies at the University of Texas at Austin involving high-resolution anatomical scans of syrinxes from hummingbirds and ostriches — the world’s smallest and largest bird species, have led to the discovery that the syrinx and larynx, the vocal organ of reptiles and mammals, including humans, share the same developmental programming. There is a scientific term for this “deep homology” that describes how different tissues or organs can share a common genetic link. The syrinx is not a modified organ with a new function but a completely new one with an ancient and common function.

Conclusion: In thinking about how birdsong sounds to the birds, a better analogy than human language or music might be dance. When we learn a dance routine, getting the sequence right is necessary for getting the moves right. Someone watching a dancer does not extract much information from the order of the moves. Instead, one is focused on the fluidity, rhythm and variety of the movements rather than any particular sequences in which they occur. 

Mimicry in birds

Birds are also some of the best mimicry specialists in nature. Birds of the Mimidae family, including mockingbirds, thrashers, and catbirds, are so named because of this family’s skill at mimicking other species. 

Toxostoma rufum, aka Brown Thrasher, dubbed the champion mimic of Noth America, can sing up to 2,000 different songs. 

Gracula religiosa, aka Himalayan hill mynas and Sturnus vulgaris, aka European Starlings, are also accomplished mimics known to include imitations of other birds, but also just about anything else, including motorcycles and tea kettles. 

Cyanocitta cristata, aka Blue Jays, can mimic several species of hawks.

Parrots are especially adept at mimicking sounds and human language. Unlike songbirds, which produce sounds by vibrating membranes in two different syrinxes, parrots have only one syrinx, located at the bottom of the windpipe. This is somewhat similar to the larynx in humans. Parrots also have long, muscular tongues that may be used in modifying sounds. Furthermore, parrots have forebrain areas involved in vocal learning and control of vocalization that are not found in other birds. 

Psittacus erithacus, aka African Gray Parrots, are one of the most accomplished mimics. A bird named “Prudle”, a male African Grey is listed in the Guinness Book of World Records as having a vocabulary of more than 1,000 words. 

References

1. Fishbein, A. (2024, February 20). What birds really listen for in birdsong (It’s not what you think). Scientific American. https://www.scientificamerican.com/article/what-birds-really-listen-for-in-birdsong-its-not-what-you-think/
2. Saini, A. (n.d.). The link between birdsong and language. BBC Earth. Retrieved July 4, 2024, from https://www.bbcearth.com/news/the-link-between-birdsong-and-language
3. Eliason, C. (2024, May 23). Birdsong and Human Voice Built from the Same Genetic Blueprint. Jackson School of Geosciences. Retrieved July 4, 2024, from https://www.jsg.utexas.edu/news/2024/05/birdsong-and-human-voice-built-from-same-genetic-blueprint/
4. Which birds are the best mimics? (2023, August 9). All About Birds. Retrieved July 4, 2024, from https://www.allaboutbirds.org/news/which-birds-are-the-best-mimics