Birds use different types of vocalizations produced in different contexts: maintaining contact with fellow birds, alarm in the presence of a predator, defense of the territory, attraction of a sexual partner…
As the human species, more than half of the species of birds are able to imitate the sounds produced by congeners. In many species, this capacity even exceeds the framework of the specific vocal repertoire: it can be sounds produced by other species, sounds from the environment, even musical melodies.
To what extent can the vocalizations produced by birds tell us about their cognitive capacities and their internal state: what are they thinking about? what do they feel?
In 1974, the philosopher Thomas Nagel published an article with the following title: “What does it feel like to be a bat?”. According to him, like Jakob Von Uexkull who preceded him in this reflection with the notion of Umwelt (the proper world of each species), we have no way of putting ourselves in the place of an animal of another species and to be able to apprehend his life experience. But the ethologist nevertheless has the tools to study communication in animals and to propose hypotheses that can be verified by experimentation, both in the laboratory but also in the field.
Before looking at the messages conveyed by vocalizations in birds, it should be remembered that there are nearly 10,000 different species. Each species represents a different story, built over hundreds of thousands or even millions of years. We must, therefore, take into account the socio-ecological conditions of each species and its evolutionary history to better understand what its representatives communicate by voice.
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Like humans, almost half of bird species are able to imitate the sounds of their social environment, mainly those produced by conspecific individuals. These species belong to three different taxonomic groups: (parakeets and parrots), Trochilidae (hummingbirds) and oscines (about two-thirds of passerine species such as the blackbird, the nightingale philomel, the canary).
Among the vocalizations produced by birds by learning, it is the song of the oscines which has been the subject of the greatest number of studies. In the Mandarin diamond, it has been discovered that only males sing and that song is learned during a sensitive period of early life, between about 25 and 90 days after hatching.
In other species like the canary, the song can change during life, from one season to another. Finally, if for some species, these changes can be spread over several weeks or even months, some species are able to modify their vocalizations almost instantly to imitate a model. Some species are also able to imitate sounds from their sound environment. The learning of human words by certain species is the most emblematic example. This is the case in many species of Psittacidae and corvids but also in certain species of oscines such as the starling.
If vocalizations have been described in all the bird species listed, some species have little or no vocalization while others have a rich vocal repertoire of several different types of vocalizations issued in different contexts.
This description is facilitated since the second half of the XX th century by the ability to record and analyze the vocal productions of birds or synthesize before the broadcast. This procedure makes it possible to test the reaction of birds to natural or artificial stimuli, and thus to try to decode the message conveyed by identifying the sounds or the acoustic parameters relevant in the transmission of the message.
The work consists first of all in describing the vocal repertoire: classifying the vocalizations according to their emission context and their semantics, sometimes using onomatopoeias and/or human words to facilitate their identification.
In 1871, in his work on human evolution and sexual selection, Charles Darwin already pointed out that “the sounds emitted by birds offer, in several respects, the closest analogy to language [… ] ”. As with human language, the vocalizations produced by birds have different functions which are not mutually exclusive. Among the language functions defined by the linguist Roman Jakobson in 1963, only the metalinguistic function (the possibility of speaking the code itself, namely language in humans) is not observed in other animal communications. The expressive function refers to the motivations of the sender and also depends on its intrinsic characteristics, on what we summarize as the internal state of the animal: its physiological state, its emotions.
The influence of emotional states
If certain vocalizations can be emitted in different situations (for example cries of contact during the communication between individuals of a social group, or cries of alarm when the detection of a predator), the acoustic structure of some screams can be changed depending on the emotional state of the transmitter. For example, in male mandarin diamonds, the spectral structure of the contact call produced in the presence of a congener is different from that of the call produced when the emitter is isolated and stressed. The diffusion of these contact calls produced by a stressed individual leads to an increase in corticosterone, the stress hormone, in the birds tested, which can be characterized as emotional contagion.
Still in the register of emotions, recent developments in neurobiology fuel reflections around pleasure. Among the Oscines, it is considered that the song, which is the prerogative of males in many species, is involved in sexual selection. Thus, depending on the species considered, the song would serve to defend a territory and/or to attract potential sexual partners.
In female white-throated sparrows, reward circuits have been observed in the brains of females when listening to songs. The activation of these dopaminergic pathways is often associated with pleasure from work carried out in humans. Furthermore, the activation of these brain circuits has been shown during the production of song in males of several species. Until now, this hedonistic dimension of song among the Oscines had been obscured by other more pragmatic aspects. For example, it has been proposed that song, by its duration or its sound composition, can be an honest indicator of the reproductive or cognitive capacities of the transmitter. But an in-depth analysis of the work carried out so far does not allow us to validate these hypotheses.
In some cases, vocalizations can also have a referential function and provide information on elements of the environment, such as the presence of predators. In the Japanese chickadee, birds produce different calls to designate different types of predators.
In the eyeglasses, individuals use separate calls to call out to each individual in the group. Many vocalizations also have a phatic function in maintaining contact, in particular in social species. Finally, other functions found in humans raise questions in non-human animals and in birds in particular: the conative function and the poetic function.
A source of inspiration
Concerning the conative function, it is the question of the intentionality of vocalizations in non-human animals which is debated. In the examples cited above (contact calls from the Mandarin diamond, alarm calls from the Japanese chickadee), these calls could be interpreted as a representation of the internal state of the transmitter, without their will to modify the behavior of the potential receiver (s) and thereby without necessarily having to lend them cognitive capacities, such as expectations or intentions.
For years, intentionality was seen as a characteristic intrinsically linked to human language. In recent years, the list of criteria to be fulfilled in order to be able to speak of intentionality in the field of animal communications has been revised in order to explore its phylogenetic aspects.
Concerning the poetic function which also refers to the notion of aesthetics, it is the song of the oscines that we can call upon to illustrate it. The song of different species has inspired many famous composers like Ludwig Van Beethoven or Olivier Messiaen: we find musicality in some of these sound productions with moments of tension and relaxation. Singing could therefore also be perceived as an artistic activity, referring to the hedonistic dimension of this signal mentioned above.
If the works on the song of the Oscines have so far highlighted the parallels with language in humans, they can also potentially fuel reflections around music, its evolution and even its phylogenetic aspects.
With their diverse and varied vocalizations, birds occupy an important place in the sound universe of the planet and we still have a lot to discover what they tell us.