How exactly do birds ‘talk’ to one another? And might research into how birds listen to birdsong help us understand communication both in humans and birds?
Dr Robert Dooling and his team, based at the University of Maryland, USA, showed that – for zebra finches at least – the subtle nuances in sound texture or timbre are more important than the sequence of repeated sounds.
Find more on their work in Research Outreach, or read the original paper : https://doi.org/10.1098/rstb.2019.0044
Hello and welcome to Research Pod. Thank you for listening and joining us today. In this episode we will be looking at the work of Dr Robert Dooling and his team, based at the University of Maryland in the US, who study the acoustic elements of zebra finch songs and their implications for human speech and language.
Researchers have been curious about birdsong for a very long time; not only from a physiological point of view about how birds can produce and recognise such sounds, but also what similarities – if any – birdsong shares with human speech. Even Aristotle in his “History of animals” noticed that in addition to learning a song from their parents, young birds can also modify the original and create a new song.
More recent studies have shown how, just like babies, young birds can learn a series of sounds and then use them in their adult life to communicate with other birds.
Despite this progress, how exactly birds ‘talk’ to other birds to convey important information is still a mystery – one that Dr Robert Dooling and his team from the University of Maryland, USA, are keen to unravel. The team in the Ball-Dooling Laboratory are experts in Behavioural Ecology, Linguistics, Psychoacoustics, and Neuroendocrinology. Together they have worked on problems from the brain mechanisms underlying vocal production to perceptual specialisations for perceiving species-specific song to the coding of meaning in sound sequences. This work may have important implications for our understanding of the evolution and unique structure of human language.
As one model, Dr Dooling and his team used Zebra finches or, to use their scientific name, Taeniopygia guttata. In this Australian bird, only males can sing. Young males have to practice for months in order to learn how to produce a song, which they largely copy from their father. For these males, there is a short window during which juvenile birds learn by imitating their father.
Zebra finch songs are made up of only 3 to 8 of distinct notes, or syllables, which are short, buzzy sound bursts with a distinctive structure. These syllables are repeated in a fixed order to make up an entire song. Zebra finches mate for life, and the male uses his song to attract a female. In addition to song, zebra finches also use many other vocalisations which are not learned. Both male and female zebra finches produce about 10 different types of calls. In spite of the fact that zebra finch calls and songs sound like ‘chatter’ to us, males and females are communicating important information, like how to coordinate parental care.
But what is more important for a zebra finch? The sequence of syllables or the fine details or timbre of individual syllables?
To answer this question, the research team trained zebra finches to detect changes in a syllable’s timbre or in its position in a sequence of several syllables.
It is tempting to assume that sequence is more important than structure for zebra finches because that’s how human language typically works: you can say words with a different stress or different accent, but the linguistic meaning is the same. However, if you change the order of syllables in a word, or words in a sentence, the meaning may completely change.
However, Dr Dooling found this is not true for zebra finches. For example, they cannot hear the difference between ‘turbo’ and ‘boater’ but can hear the smallest changes in the acoustic structure of the syllables ‘bo’ and ‘tur.’ Zebra finches also struggle to detect changes in the sequence of syllables in their own song but can easily notice the smallest changes in the fine structure or timbre of a syllable.
Dr Dooling explains: “Fascinatingly, zebra finches, and other birds, outperform humans at detecting these very tiny changes in the acoustic structure of syllables, performing well beyond human capabilities but at the same time, struggle to hear changes in the order of syllables in the song – a task that is easy for human listeners.”
If birds had experience with the song beforehand, they were a little better at recognising a change in the sequence of syllables, but they were still much more sensitive to changes in syllable acoustic structure. It’s not unreasonable to speculate that syllable sequence may have some communicative significance between partners, but it looks like the real message is in the subtleties of syllable structure. The research team say: “These results provide potential insight on the significance of zebra finch song features and suggest new avenues of investigation in using zebra finches and other birds as animal models for the evolution of sequential sound processing.”
This work reminds us that human hearing may not be the best tool to understand what aspects of a birdsong are important for birds. It’s entirely possible that, if we cannot hear these differences, we’ll never truly understand birdsong. Dr Dooling says: “The fact that the sensitivity of birds to the minute structure of individual song elements well surpasses that of humans strongly suggests that birds are communicating important information using an acoustic channel that is beyond human perceptual capabilities – reminiscent of the ultrasonic echolocation calls of bats that are beyond the range of human hearing.”
At the very least, these results suggest acoustic communication between zebra finches – and possibly many other birds – relies heavily on subtle timbre features of notes and syllables. In other words, syllable sequence may be a surface parallel between birdsong and human language, but the structural nuances in the syllables are much more critical for birds in voice recognition and communicating important information.
To further complicate matters, not all birds behave like zebra finches. While zebra finches learn one song when they’re young and keep it for their entire adult life, other species, like the budgerigar or Melopsittacus undulatus, learn throughout life and perform with more variability in their songs, more akin to vocal learning and communication in humans.
Interestingly, faced with the same challenge as zebra finches, budgerigars had no problems detecting changes in the sequence of syllables. These results are curious, to say the least, but Dr Dooling cautions that is too early to tell whether this is a real difference between some species of birds and more research is needed. He says: “Whether our findings represent a songbird-parrot difference or a species difference in fixed/flexible song learners cannot be addressed with only two species. However, these results do suggest an interesting avenue for further comparative investigation on the neural basis of fine structure and sound sequence processing in birds.”
One aspect of the zebra finches’ ‘chatter’ that sparked Dr Dooling’s curiosity was what kind of information was hidden in the timbre of their vocalisations. Not only do zebra finches mate for life, but they are native to Australia where weather is unpredictable. Zebra finch pairs don’t have a fixed breeding season but breed only when weather is good, and they may travel large distances in search for water and food. For this arrangement to work, partners have a lot to coordinate with one another (such as where to go, how stressed they are, and when to breed).
Seeing similarities between birdsong and human language may be very appealing, but Dr Dooling’s work highlights critical differences between the systems. The team believes that zebra finches, and probably birds in general, may be communicating sophisticated individual and social information in the acoustic details of calls and songs to which the human ear is largely insensitive.
Despite these differences, birdsong can still provide valuable information to understand human language. The differences between zebra finches, and say budgerigars, in how syllable sequences are perceived, can be used as a roadmap to study how the vertebrate brain encodes and recognises learned vocalisations. Studying perception of song syllables also provides the opportunity to understand how biologically critical information is conveyed in subtle features of voice for both birds and humans.
That’s all for this episode – thanks for listening, and stay subscribed to Research Pod for more of the latest science. See you again soon.
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