By Jerry Lang
Every animal has its own unique communicative abilities to interact with others of its species and with the environment. Sound in the form of songs and calls is vitally important to songbirds. Good birders can bird as much from hearing species-specific songbird songs and calls as from actually seeing the birds. The hearing “sweet spot” for humans and birds is similar – 1000 to 8000 Hz. But does that mean we really hear what birds hear?
We assume birds have been singing for at least 67 million years, which is the age of the first fossil syrinx. The syrinx is an organ unique to birds. It is located where the windpipe splits to the lungs and has rings of muscle that enable the repertoire of birds sounds. These muscles contract faster than any other vertebrate muscle, facilitating millisecond control. The duel nature of the syrinx allows birds to sing two notes at the same time and to change pitch extremely fast – something that animals that have larynxes only cannot do. Evolutionarily the syrinx is unusual in that it did not seem to slowly evolve from another structure, but just suddenly appeared in the fossil record. Recently, scientists reported that ankylosaur dinosaurs (the last of the non-avian dinosaurs, living 68 – 66 million years ago) may have made bird-like vocalizations with their larynx, but these non-avian dinosaurs did not have a syrinx.
Equipped with a syrinx, birds have the capability of producing a rich repertoire of sounds, which may have helped the development of speciation over the millennia. The ability to produce sound variation also assumes the ability to hear the details of such variation and learn what that variation means.
Bird hearing works much like human hearing although birds lack an outer ear. Bird ear openings are usually below and slightly behind the eyes and with a covering of feathers called auriculars. The middle ear sends vibrations from an eardrum to the columella and then to the cochlea where fine hairs in fluid are activated and send stimuli to the brain where sound is interpreted.
Birds hear the gross temporal features and possibly the syntax of their songs like we do. However, bird hearing works on a much faster scale than does ours. Experiments have shown that birds can detect individual clicks 1.3-2 milliseconds apart at speeds of 500-770 clicks per second. Humans can only detect clicks longer than 3-4 milliseconds in length. As a result, birds can hear much finer sound structure than we can – all the chips, chirps, twangs, and other variations inaudible to us.
While the sequences in birdsong are important to us in identifying birds, they may often be irrelevant to the birds that pay more attention to the fine structure of their singing. This might be somewhat like if we listened to each other speak and paid more attention to the vowels we used rather than the words or their sequence! Birds’ ability to hear pitch as well as fine structure (speed) can vary by season and sex. As an example, in the fall, black-capped chickadees are more sensitive to speed than to pitch. This reverses during spring breeding when males need to render pitch as pure as possible.
Beyond humans, dolphins and a few other animals, birds are unique in that they have the capacity for vocal learning. Humans and birds have analogous brain structures unique to learning vocalization, making some birdsong roughly similar to a language with phonology (the pattern of sounds making up words), semantics (the connection of words with meanings), and syntax (the order and combination of words). Both bird song and language can be culturally transmitted. Many nestlings and young birds go through a learning process by listening to songs of their parents or other nearby conspecifics. They go through a stage called ‘sub-song’ which is the equivalent of human infant babbling. After often several months of practice, young birds produce a ‘crystalized song” that has all the spectral and temporal characteristics of the species. However, the culturally transmitted songs can lead to small song variations, which over time, can develop into localized or regional “dialects.”
Although many temperate region female birds do not sing, many females of tropical birds do sing, and males and females engage in duetting. Exactly how birds that duet learn and coordinate this behavior is not well understood.
Some birds can learn new vocalizations beyond their species-specific songs and calls. These mimics like mockingbirds, parrots, nightingales, ravens, starlings, etc. are very “plastic” in their abilities to hear and learn new songs/sounds. In some cases, a male songbird’s ability to mimic as well as sing its own song is an important factor in sexual selection by females.
Many songbirds also have meaning in call variations (simpler vocalizations than songs) used for alarm in different situations, and calls are often used to communicate with a mate while out of sight or to gather others to mob a predator, etc. Calls are also used by migratory species to communicate at night and maintain contact with each other. Apparently in communally nesting species, parents can often identify their own young from hundreds of others by unique calls.
So do songbirds have a vocal language? There’s a strong similarity between birdsong learning and human speech both of which rely on similar brain mechanisms and developmental constraints. It’s probably safe to say that songbirds have a limited language as defined by linguists. However, we don’t know and have yet to detect all that birds are “saying” to each other through both what we can hear and what we can’t hear. As much as we study bird vocalization and brain physiology, we will probably never truly know the kind of sound world in which songbirds live.
First published in Desert Rivers Audubon Magazine
Photo: Western Meadowlark by Vida Ward, Audubon
