How Do Birds Learn Their Songs? A Critical Period and a Tutor

Song development follows a sequence that parallels human speech closely enough to be uncanny:

• •A sensory phase early in life, during which the young bird listens to and memorises a tutor song, usually its father or a neighbouring male, without producing anything itself
• •Subsong, a quiet, rambling, variable output resembling human infant babbling, in which the bird experiments with sounds
• •Plastic song, in which recognisable elements of the memorised model appear and are rehearsed and gradually refined, with the bird listening to its own output and correcting it
• •Crystallisation, at which point the song becomes stable and, in many species, changes little for the rest of the bird's life
• •A critical period bounds the sensory phase, so a bird that hears no tutor within that window never produces normal song, and the window's length varies from weeks to most of the first year depending on species
• •Deafening a bird after it has memorised a model but before it has rehearsed prevents normal song from developing, which shows the bird must hear itself to match its own output to the memory

Vocal learning is not general across birds. It is found in three groups: the songbirds, which are the oscine passerines and number several thousand species; the parrots, which learn throughout life and can imitate sounds far outside their own repertoire; and the hummingbirds. Other birds, including pigeons, chickens and most non-passerines, produce their calls innately and do not learn them. Within songbirds the strategies vary enormously. Some species have one song and learn it once. Others, including mockingbirds, starlings and lyrebirds, are open-ended learners who continue adding material lifelong, which is why a lyrebird can reproduce other species, chainsaws and camera shutters, and why a starling can imitate a telephone. Repertoire size ranges from one song type to thousands, and in several species females prefer males with larger repertoires, which is thought to drive the elaboration. Regional dialects arise because young birds learn locally, so populations separated by distance diverge in song, in some cases sharply enough that neighbouring valleys differ.

Birds have no larynx in the mammalian sense and sing with the syrinx, a structure at the point where the trachea divides into the two bronchi. That position gives it two independently controllable sound sources, one on each side, and several species use them separately, producing two notes simultaneously or alternating breaths between sides to sing without pausing, which is how a thrush can produce apparently impossible continuous phrases. Control comes from muscles that are among the fastest in vertebrates, adjusting tension hundreds of times a second. The learning is supported by a dedicated set of brain regions collectively called the song system, whose discovery made the songbird a workhorse of neuroscience: one pathway is required for producing learned song, another for learning it, and lesions to the second impair acquisition while leaving an already crystallised song intact. Work on these circuits produced the finding that new neurons are generated in the adult brain, which overturned a long-standing assumption about vertebrate brains generally.

Song serves two overlapping functions and both explain why it is loud, repetitive and costly. It advertises territory, warning rival males that a holder is present and in condition, and experiments playing recorded song into an emptied territory delay its occupation by intruders, which shows the signal alone does work. It also attracts and stimulates females, and features including repertoire size, song rate, the accuracy of difficult passages and the consistency of performance function as indicators of the singer's quality, since a bird that was stressed or underfed during its learning period sings measurably less well, making song an honest record of early life. The dawn chorus concentrates singing when sound carries furthest in still cool air and when foraging is least productive. Human noise interferes directly with all of this, and urban birds have been documented singing at higher pitch, at greater volume and at different times to avoid being masked by traffic, with measurable costs to how attractive those songs are.

Songbirds, parrots and hummingbirds learn their songs rather than inheriting them, memorising a tutor during a critical period, then babbling, rehearsing and refining until the song crystallises, which requires hearing themselves. Other birds produce calls innately. The syrinx has two independent sound sources, allowing two notes at once. Song advertises territory and signals quality honestly, since poor early conditions degrade it permanently. Research on the song system revealed that adult brains generate new neurons.