Daily Nature Diary 3: Birdsong

My friend Mollie asks: “why do birds that hang out near water have ‘uglier’ calls than the birds we like to watch congregating in our trees?”

Mollie lives in downtown Chicago, just a few blocks away from Lake Michigan. To break up the days stuck inside, she’s been running on the beach and watching the squawking flocks of Herring gulls (Larus argentatus) roaming the empty shores. Meanwhile in Virginia, I awoke to the dawn choruses of Northern cardinals (Cardinalis cardinalis), Carolina wrens (Thryothorus ludovicianus), Eastern bluebirds (Sialia sialis), and Song sparrows (Melospiza melodia).

It’s an excellent question. Why do songbirds sound prettier than shorebirds? In response, I’ll consider three different perspectives, all fascinating, all non mutually exclusive.

Comparative Morphology

Are there anatomical differences between shorebird and songbird vocal parts that give their voices different sound qualities? To make sound, birds have a unique organ called a syrinx, which is located lower down in the windpipe than the larynx. Birds also have larynxes like most tetrapods, but their larynxes no longer function in sound production. A bird’s syrinx branches into two pipes, one leading to each lung. Across the avian tree, there is ample variation in the size, shape, and structural complexity of syrinxes to create a phylogenetic symphony of squeaks, whistles, warbles, chirps, screams, croaks, or in rare cases, no sounds at all.

Oscine songbirds, like the crooners out my window, have the most complex syrinx structures of any modern bird. Their syrinxes are tracheobranchial, extending all the way from the lower trachea to the upper bronchi of the lungs. The gulls Mollie passes on her run also have tracheobranchial syrinxes, but theirs have only two to four muscle attachments vibrating the cartilage to make sounds. Oscines can involve up to six different sets of muscles in music making, some connected directly to the cartilage and others extending from their tongues, throat, lungs, and even their upper wing muscles. Some songbirds can even vibrate the right and left branches of their syrinxes independently, letting them sing two distinct tones at once.

Hence, shorebird voices may be less versatile and flexible than songbird voices. Fewer muscle attachments afford a narrower range of vibrations. I’m imagining the grating twang of an over tuned guitar string or piano wire compared to one that vibrates freely. However, some songbirds sound like they have severe laryngitis, or should I say, syringitis. Take the American Robin (Turdus migratorius) versus the Scarlet Tananger (Piranga olivacea). Their tunes are very similar, but their vocal qualities are like Ella Fitzgerald and Louis Armstrong. I don’t think we’ve thoroughly compared the syrinx structures and their respective acoustic physics for enough songbirds to know exactly why their voices sound so different. Why do Louis’ and Ella’s, for that matter? If you showed an ornithologist only the spectrograms of both legends singing one of their lovely jazz duets, she might say they were different species!

Life History/Behavioral Ecology

I’ll admit that anatomical differences were not the first idea that sprung to mind. As a behavioral and evolutionary ecologist, I’m preoccupied with how gulls and robins use their voices for different kinds of communication. Still, it seemed appropriate to start with how their instruments differ before we get into why these differences may have evolved. Any satisfying explanation in biology combines both structural and functional perspectives. The why question requires an investigation of their natural histories.

The question of use is tricky because, well, we don’t speak bird. The cardinal in my dogwood tree sings, whistles, chips, chatters, and clicks in the course of an hour. We know from decades of careful observation that different sounds are associated with different contexts, in turn suggesting different meanings. Shorter, less musical tones are often described as warning calls, threatening rivals in the area to stay away from their territory or alerting others in the woods to a nearby predator. Longer songs are usually interpreted as mating calls to help birds recognize and select mates. An individual songbird might use three different variations of its courtship song, and different populations of the same species may have distinct local dialects.

In contrast, gulls don’t say anything we’d generously call song, but they do give distinctive courtship squawks that sound like guffawing laughter. We also haven’t studied gull voices nearly as much as songbird voices, probably because our hearing isn’t sensitive to pick up on any subtle variations in a gull’s screams, though they’re almost certainly there. From recent analyses of individual gulls’ spectrograms, we now know that individuals make a distinct call that may serve as their identifier within a flock: I’m Keeeeeeow, and this is my brother Keeooooow.

I think part of this answer comes down to differences in sociality that affect how gulls and songbirds use their voices. Gulls live and nest in large social groups year-round, so their incessant blather may be focused on conveying quick information to the group: “look, french fries!” “Watch out for the breaker!” “Hey, that’s my fish!” Most songbirds set up separate nesting territories during spring and summer and sing mostly to attract and communicate with their mates. During the non-breeding season, they spend more time in small family groups or occasionally larger feeding flocks. Songbirds in wintering flocks often chatter to each other with shorter, more gull-esque quips. The brain regions associated with song learning and recognition even grow and regress from spring to fall in some songbird species, indicating that a songbird’s investment in song and voice use varies dramatically across the year.

Properties of their environment may also affect the evolution of shorebird and songbird sounds. Gulls congregate anywhere people and their trashcans do, but their ancestral environments are windy shores and open oceans. The roar of the ocean surely affects their hearing abilities, perhaps forcing them to scream to compete with the crashing waves. If screamers are better able to communicate with mates and social partners, over time selection may have pushed all gulls to sound like ACDC. (*This is pure speculation on my part.) We know songbird species that live in more open habitats tend to have different vocal qualities than songbirds who live in more forested habitats, partly because sound waves travel differently and face different levels of interference in these disparate acoustic environments. The birds around my neighborhood don’t have to shout over lots of ambient noise of project their calls through a lot of obstacles. Perhaps this means they can afford to be a bit more musical, though they’re still plenty loud. Looking at you, Mr. Cardinal.

Music Philosophy

Finally, I’ll take a more philosophical perspective on the question of song ‘beauty.’ All preferences are subjective, whether genetically encoded or socially transmitted. We think songbirds sound prettier than shorebirds because their voices are more similar to what we’d consider music. However, birds certainly don’t sing for our pleasure. They sing for each other. If there is an overlap in taste, it’s purely coincidental. My zebra finches (Taeniopygia guttata) used to go wild when I played them saxophone music in lab, I think because they thought it sounded like another finch.

Still, maybe evolution has shaped songbirds’ melodic ‘beauty’ for their own attractiveness, as Darwin believed. Songbirds might sound prettier when they sing because they’re trying to impress their partners. The better they sing, the more likely they are to breed, so there’s a pretty strong incentive to perform.

Voice quality doesn’t appear to be an important factor for shorebird breeding success given that they don’t sing complex melodies like songbirds do, but then how would we know? Maybe gulls think their shrieking is the most beautiful music on Earth. They must given how much they talk, otherwise how could they stand to listen to each other all day long? They’d probably find the little whistles and warbles of songbirds highly offensive, just like the older generation hates the younger generation’s ‘garbage music.’ One bird’s trash rock is another bird’s treasure.

I hope this helps answer your question, Mollie, or at least gives you a lot of interesting things to think about. Thanks for giving me such a fun puzzle for a slow Sunday morning!

References for Syrinxes:

  1. Kingsley, E. P., et al (2018). Identity and novelty in the avian syrinx. Proceedings of the National Academy of Sciences115(41), 10209-10217.
  2. Suthers, R. (2004) How birds sing and why it matters. In Nature’s Music: the science of birdsong. Marler, P, & Slabbekoorn, H. eds. Elsevier. New York. Chapter 9, p. 272-295.
  3. Riede, T., & Goller, F. (2014). Morphological basis for the evolution of acoustic diversity in oscine songbirds. Proceedings of the Royal Society B: Biological Sciences281(1779), 20132306.
  4. Brown, C. & Ward, D. (1990). The morphology of the syrinx in Charadriiformes (Aves): Possible phylogenetic implications. Zoological Bulletin, 41(2), 96- 106.
  5. Warner, R. W. (1972). The anatomy of the syrinx in passerine birds. Journal of Zoology168(3), 381-393.
  6. Ince, N. G., Pazvant, G., & Alpak, H. (2012). Anatomical features of the syrinx in sea gulls. Ankara Üniversitesi Veteriner Fakültesi Dergisi59(1), 1-3.

References for Behavioral Ecology

  1. Brumm, H., & Naguib, M. (2009). Environmental acoustics and the evolution of bird song. Advances in the Study of Behavior40, 1-33.
  2. Mathevon, N., Charrier, I., & Jouventin, P. (2003). Potential for individual recognition in acoustic signals: a comparative study of two gulls with different nesting patterns. Comptes Rendus Biologies326(3), 329-337.
  3. De Groof, G., Verhoye, M., Poirier, C., Leemans, A., Eens, M., Darras, V. M., & Van der Linden, A. (2009). Structural changes between seasons in the songbird auditory forebrain. Journal of Neuroscience29(43), 13557-13565.

All sounds courtesy of the Cornell Lab of Ornithology’s Macaulay Library. Featured image is a screenshot from Mollie’s video.

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