In humans, as with virtually all other terrestrial vertebrates, sound is produced in the larynx aka the voice box. As air passes through the larynx it vibrates the vocal cords, producing sounds. Across different species this system is enhanced or reduced, resulting in the roar of a lion, the grunt of an alligator or that conversation you wish you could have avoided with your coworker. While birds also have a larynx, it doesn’t produce sound. Instead, birds (well, most birds) have a wholly unique structure called the syrinx which sits not at the top of the trachea as the larynx does, but at the bottom, right at the bronchial split. This unique forking anatomy allows some birds to lateralize their sounds, meaning making different sounds on the left or right side, sometimes even at the same time! The repertoire of parrots is further enhanced by their fleshy (for a bird) tongue, which can manipulate air flow and produce more human-like speech.1 It’s this level of vocal complexity and control that possess birds with the incredible vocal range we hear, whether they use it for mimicry or not. In fact, despite this potential, most songbirds do not mimic, and actually cannot learn new songs after their first year of life.
But while the syrinx explains why some birds can produce human speech sounds, it doesn’t explain why our closest primate relatives cannot, especially given the similarities of their voice box with our own. That’s where the brain comes in. While non-human primates have the correct hardware in their vocal tracts2, they’re missing the technology they need in their brains.3 Specially, in the cortical association areas in the neocortex (the part of the brain that’s responsible for our higher-order behaviors). They simply don’t have the neurological control required to mimic human speech (though they do mimic us in other ways). So why do birds?
Unlike in primates, vocal mimicry is a cornerstone of communication and signaling in certain birds. Whether they’re using it to advertise their quality as a potential mate, territory defense, or as a way to bond with partners or group mates, mimicry plays a key role in engaging with those around them. Though the specifics of that engagement may not always be clear. For example, despite persistent online assertions that Steller’s jays mimic hawks either to warn of their presence or to scare competitors, there’s zero evidence in support of either of these. In fact, a study from 2017 found that wild jays almost never do it when predators or competitors are around, and instead do it most often at the beginning of the breeding season in front of their mate.4 Which is probably for the best. After all, if corvids chose to really make a habitat of using mimicry to trick other animals we’d likely find ourselves at the top of their target list.
Literature cited
1. Beckers G, Nelson B, and Suthers R. 2004. Vocal-tract filtering by lingual articulation in a parrot. Current Biology 14: P1592-1597
2. W. Tecumseh Fitch, de Boar B. Mathus N, Ghazanfar A. 2017. Monkey vocal tracts are speech ready. Science Advances 2: DOI: 10.1126/sciadv.1600723
3. Dunn J and Smaers J. 2018. Neural Correlates of Vocal Repertoire in Primates. Frontiers: https://doi.org/10.3389/fnins.2018.00