Seeds for Thought: Is there Structure in Birdsong? We may be underestimating the communication systems of birds.

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A group of alien linguists receives a modest grant to do fieldwork on the communication systems of animals on Earth. They notice two groups making a lot of noise: birds and humans, and since there are a lot fewer humans than birds, they decide it’s more feasible to focus on the featherless bipeds. They record all the different sounds we make, the order in which they are usually produced, and who we’re with and what’s around us when we converse. After analyzing terabytes worth of data, they gain some compelling insights into our communication system: they discover phonological rules, dialects, vowel formants, and categorical perception. They posit reasonable hypotheses about the function of language in building alliances, courting mates, and managing conflict. But, I would argue, following Chomsky, that they’d be missing out on fundamental properties about the nature of language.

For one thing, since the relationship between sounds and meanings is essentially arbitrary, they would have a difficult time uncovering what our words mean or even which segments of the sound signal are functioning as words. On top of that, without access to meaning, they would have very little chance at discovering rules and principles of how words fit together, i.e. syntax. As a result, these alien linguists would miss out on the structure of human language: how words combine to form phrases that can join in infinite ways with other phrases. Despite having only studied the surface of our language, they would conclude that these humans lack the infinite generativity and recursive rules of their own beautifully unique alien language.

After decades of humans studying the communication systems of birds, the predominant consensus is that birdsong is all surface: no compositional meaning, no hierarchical combinations of phrases, only strings of sounds. And, while I agree that there is no strong evidence of structure below the surface in birdsong, I would argue that those alien linguists, without our intuitions about how words fit together, without our judgements of what constructions are acceptable and not, would be hard pressed to find evidence of structure below the surface in human language.

So, when it comes to birdsong, are we the alien linguists? Are we missing out on structure below the surface?

Budgerigar (Melopsittacus undulatus) flock. Originally posted to Flickr by anna banana and licensed under the Creative Commons Attribution-Share Alike 2.0 Generic license.

Traditionally, researchers have analyzed the sequences of sounds produced by songbirds and parrots by using concepts from language and music: a note is a continuous trace on a spectrogram, a syllable is a collection of notes, and a motif or phrase is a grouping of syllables. They then characterize patterns in the order of syllables or phrases by using Markov chain models. These models capture how the probability of an element occurring in a sequence depends on the previous elements. If the two foregoing elements help to predict the next one, then it is a 2nd order Markov model. If three elements, then 3rd order, and so on. Most birdsong can be characterized by low (1st or 2nd) order Markov models. Vicky Hsu, a former PhD student at UMD, studied the complex and variable warble song of parakeets and found that it can arguably be best captured by a 5th order Markov model. This is very impressive but still categorically different than the hierarchical depths of human language.

Yet, think about this: if you just studied the sound or sign patterns of human language, then you could also characterize them with Markov models, as our own Bill Idsardi has argued. You would have no idea, of course, that there is structure below the surface. Like the alien linguists, you would confidently assert that human language patterns are Markovian in nature, easily computable by finite-state machines. Could that also be the case for us and the birds?

In my view, this question is still open and important. Uncovering the putative structure of birdsong would give us an unparalleled window into the minds of birds, possibly helping us to better understand the workings of the computational devices inside all of our skulls (whether bird or human). But needless to say, it is not satisfying just to claim that structure in birdsong is possible. We want to find evidence either way. Here are two possible steps forward I think we could take:

  • While there is no evidence for anything like words in birdsong in our studies of syllables and motifs, we might be wrong about the fundamental units in their communication systems. Recent studies have shown that songbirds and parakeets are exquisitely sensitive to temporal fine structure (TFS) — rapid changes in the amplitude and frequency of the acoustic waveform. Their perceptual abilities in this dimension actually exceed our own: zebra finches, for example, can hear changes in TFS for periods as short as 1-2ms while humans require periods 3-4ms long. This means that birdsong may sound very different to the birds than to us. Thus, while many birdsongs appear simple and repetitive to us, more complex patterns and variability could be embedded in the strings. Examining temporal fine structure patterns and understanding perception in this acoustic dimension could help us uncover any hierarchical structure, if it exists.
  • While we are stuck looking at strings of sound when analyzing birdsong, there are tools out there to help us decode structural rules governing the strings. Much like we can make up minimal pairs of sentences and ask human subjects to make acceptability judgments in order to test hypotheses about grammatical dependencies, we can ask the birds to “judge” strings of sounds as valid and invalid. This is not an easy road to take, since, as I’ve argued, we lack the intuitions of a native speaker for what elements might fit together. But we can draw on clues to structural rules: for example, in parakeet warble song, “contact call-like” elements are the most common, perhaps playing a role like function words in human language, in which case they may mark the beginning or ends of “phrases”. When it comes to asking the birds to “judge” strings, the path is a bit more straightforward. We can, for instance, ask birds to perform a preference task: you put two perches in a cage and when a bird stands on one it triggers a set of sounds manipulated in one way (with, for instance, hierarchical embedding), and when it stands on the other it triggers sounds manipulated in another way (for instance, a random arrangement of the same sounds). If a bird prefers one set of sounds over another, this could tell us what strings are acceptable and allow us to uncover any grammar in birdsong, again if it exists.

These are, of course, only two ideas of many possible paths forward. What we really need is linguists with theoretical insights, computer scientists with powerful algorithms and processors, biologists with expertise in animal behavior and cognition, birdsong researchers with innovative behavioral testing paradigms and other passionate folks to come together and approach this question with as open of minds as when it comes to presupposing the complexity of our own thoughts. My view could certainly turn out to be totally wrong. However, I’d rather run the risk of being wrong than to miss out on an entire dimension of animal cognition and communication, which could help us better understand our own. I, for one, would like those alien linguists to come back with more colleagues and new ideas (and perhaps a generous new grant) to take a chance at delving below the surface of our language and minds.

Adam Fishbein is a PhD student in the Neuroscience and Cognitive Science program at UMD, using comparative work with birds to study the evolution of human language and cognition. He also has a Master’s in Professional Writing from USC and has published several short stories and a novel.

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