You’re more of a bird brain than you think
Whenever I tell someone my interest is birdsong, their usual response is a direct “Why?”. The link between birds (taxon Aves) and humans is not straightforward: our last common ancestor would effectively be the LCA between reptiles and mammals. This is estimated to be approximately 312 MYA, as calculated by averaging across 22 available molecular analyses.
NB: For anyone interested in learning more about the tree of life I can’t recommend http://www.timetree.org/ enough. It lets you search pairwise divergences without having to know the latin names of species (thank god) and provides links to the studies the date is derived from for easy appraisal.
So, it’s been a long time. A common conclusion is that birds are so radically different from humans that it’s pointless to compare the two. Whatever similarities there are would have arrived by convergent evolution, analogies as opposed to homologies. But, that is really useful! If we find similarities between the two species, as we do, then birds are an opportunity to study which conditions lead to advanced cognitive abilities which would fall under the broad faculty of language. Much of the following information has been summarized from a fascinating paper by Beckers, 2013.
The exact mechanisms of how birds produced song were shrouded in mystery until a few decades ago. Many languages around the world use whistle as the verb used for bird vocalization, but in fact birds use a source/filter model just like the rest of us tetrapods. Pressure waves are generated by passing air through a membrane (located in the larynx in humans), which vibrates passively. In human males, the wave cycles at a fundamental frequency of about 100 Hz, and 200 Hz in females. This source sound is composed of a harmonic stack of overtones at integer multiples of the fundamental frequency. The harmonic stack is then filtered by the shape of the vocal tract and its articulators. Birds also produce additional sounds from articulators, just as we use our tongue, palette and teeth, although this has not been well studied. Humans use the vocal tract as a formant filter. Most linguistic content is encoded in the shape and movement of the formant envelope: we can whisper any sentence we wish without losing comprehensibility. In almost all birds, the voice and formants operate together. However, parrots operate them separately, allowing them to imitate human speech. The amplitude, or loudness, of birdsong is a result of increased airflow or glottal resistance. Pitch is determined by muscles which alter the tension of the membranes in the vocal organs. Despite the similarities, the physiology of birds’ vocal tract differs from mammals in a few striking ways.
The bird equivalent of a larynx is the syrinx, which sits deeper than most other tetrapods and attaches to both bronchial tubes leading to the lungs. Each bronchial connection has a labial membranes, equivalent to our vocal chords. These two labia can be controlled independently, allowing birds to produce two pulse waves at once. Importantly, song birds exercise direct control of their syrinx and respiratory muscles via the RA nucleus of their telencephalon, which allows for fine tuned motor events. Chickadees take advantage of this to produce notes which are the sum/difference of two waves of different frequencies. Some birds phonate with one labia, while reserving the other for taking breaths. Anyway, the resultant pulse waves are then smoothed into a pure sinusoidal tone in many species, which is what had tricked humans into thinking they whistle. An air sac surrounds the syrinx, allowing birds to control the surrounding air pressure which may affect pitch. It probably also acts as a resonator. There are additional non-linear interactions between membranes of the vocal tract which are not necessarily under the direct control of the bird, but lead to very interesting sonic results including high frequency modulations >500 Hz (probably impossible by neural control), transitions from periodic to aperiodic vibration, and frequency jumps.
One of the most obvious similarities between humans and birds is the presence of socially learned, structured vocalizations. Juvenile birds learn from their parents, and chicks progress through stages of learning similar to a human child — first a silent phase, then babbling, then crystallization. Individual birds raised in social isolation will produce an aberrant song and fail to learn normal song after the critical period has passed. The development of the bird’s brain must accomplish similar sensory motor transformations and mappings as children. We can assume that very similar cognitive structures necessary for vocal learning are present in both birds and humans, such as sequential memory, a phonological buffer, and motor/memory storage of vocal tract configuration.
Birdsong is organized hierarchically, with elements comprising syllables, which comprise phrases, which comprise bouts, which comprise a song. These units are not combined randomly, but are governed by syntactic rules which can be modeled using a Markov chain with transition probabilities. There are illegal, ungrammatical bird songs, the same way there are illegal sentences in a given language.
The only thing missing from bird vocalization is semantic content, although there’s growing evidence that parrots and crows might encode semantic content. Information about potential threats appear to propagate through networks of crows without a physical presence of the threat required. This suggests the “uniquely human” design feature of language displacement may be present in crow-talk. However, usually birdsong is used for socializing, territory defense, attracting mates, and calling bird brothers to arms against an enemy. Many scientists believe the vocal talents of birds were driven by sexual selection, as an indirect signal of fitness, although this is not universally agreed on.
Perhaps the most damning (in a good way) evidence that birds and humans are doing something fundamentally similar is neural and molecular evidence from brain studies of birds. In all living organisms, genetic expression is constantly changing in reaction to the surrounding environment. An obvious example is the production of melanin after long term sun exposure. This expression in your skin happens slowly compared to the brain, where genetic expression can even change from minute to minute depending on the task at hand.
The task of vocal learning triggers a significant change in the neurons’ expression of certain genes. We now know that the same genes linked to vocal learning in humans are likewise expressed in birds in a case of convergent evolution. One important, often written about shared gene is FOXP2. This gene is linked to speech and learning deficits in humans. Dysfunctional FOXP2 can impair a birds vocal abilities.
Another important pair of genes are referred to as the SLIT-ROBO molecular pathway. In humans, SLIT and ROBO dysfunctions are associated with autism, dyslexia and other speech disorders (Bates et al., 2011). ROBO1 has specific enriching mutations in the human lineage when compared with other primates. This pathway serves to guide axons from one part of the brain to the other, and appear to help connect the vocal learning circuits of the brain. Species of birds which vocally learn have a down-regulation of SLIT and an up-regulation of ROBO1 during juvenile development in the RA analog area of the brain, which is the part which makes the direct projection to the brainstem vocal motor neurons. Species which do not sing show no significant specialized regulations of these genes (Wang, Chen and Hara, 2015).
Birds and humans seem to be a good case of nature reusing the same tools to accomplish similar goals in radically different species, which is amazing but entirely unsurprising given the deep conservation of genetic information. I expect studies of other vocal learning species will implicate similar gene expression profiles. Despite it’s promise, the literature on birds is sparse and underfunded, and much more attention is needed for our feathered friends.
Bibliography
Bates, Timothy C., et al. “Genetic variance in a component of the language acquisition device: ROBO1 polymorphisms associated with phonological buffer deficits.” Behavior genetics 41.1 (2011): 50–57.
Beckers, Gabriël JL. “2 0 Peripheral Mechanisms of Vocalization in Birds: A Comparison with Human Speech.” Birdsong, Speech, and Language: Exploring the Evolution of Mind and Brain (2013): 399.
Wang, Rui, et al. “Convergent differential regulation of SLIT‐ROBO axon guidance genes in the brains of vocal learners.” Journal of Comparative Neurology 523.6 (2015): 892–906.
