Birdsong learning in many songbird species occurs in distinct stages, similar to how human infants learn speech. Song learning begins soon after hatching and continues as the bird matures. There are two main phases of vocal learning: sensory learning, which happens first, and sensorimotor learning, which happens later.
Sensory learning phase
The sensory learning phase occurs early in a bird’s development, before it is capable of producing full vocalizations on its own. During this time, the young bird listens to and memorizes the songs of adult birds around it, typically its father or other males. This sets up a neural template of the desired song in the bird’s brain that it will later try to match with its own vocalizations.
Sensory learning begins within the first few weeks after hatching. Nestling birds start paying attention to the songs of adults and may attempt to make quiet vocalizations of their own, called subsong. The critical or sensitive period for sensory learning is thought to end around 25-60 days of age, after which birds can no longer accurately learn new song templates from other birds.
During this phase, young birds are forming memories of songs they’ve heard using auditory feedback pathways in the brain. Key auditory regions involved in sensory song learning include:
- Primary auditory areas that process the spectral and temporal elements of sound.
- Higher-order auditory areas that encode more complex song properties.
- Song control nuclei HVC and LMAN, which create neuronal representations of the tutor song.
Sensory learning allows birds to develop a neural representation of the song they aim to sing before producing complex vocalizations of their own.
Sensorimotor learning phase
After the sensory learning phase, birds then go through sensorimotor learning as they begin practicing and refining their own song. This phase involves the integration of auditory feedback pathways with motor control systems that coordinate the vocal muscles.
Sensorimotor learning can be further divided into the following stages:
Subsong
Subsong occurs starting around 25-50 days after hatching in many songbird species. During this time, young birds begin soft, rambling vocalizations that lack clear song structure. These early vocalizations allow birds to start exercising their vocal muscles and establishing neural connections between auditory and motor regions of the brain.
Plastic song
After subsong comes plastic song, beginning around 50-80 days after hatching. Now birds start attempting to match their vocalizations to the template memorized during sensory learning. Plastic song is highly variable from rendition to rendition as birds are still practicing. The timing, pitch, and structure of syllables is inconsistent.
During this stage birds rely heavily on auditory feedback to shape and improve their song. If auditory feedback is blocked during plastic song, birds will often produce highly abnormal songs as adults. The ability to use auditory feedback to alter vocalizations declines at the end of this stage.
Crystallized song
The final product of sensorimotor learning is crystallized or stable adult song. This is achieved between 80-250 days after hatching, varying across species. Crystallized song matches the memorized template at a syllable-by-syllable level. The adult song has consistent acoustic features and is resistant to changes, even if auditory feedback is altered.
Some evidence suggests that crystallized song is maintained by a motor pathway involving the song nucleus HVC and vocal motor neurons. Auditory feedback becomes less important for maintaining stable vocal output.
Birds may continue to make minor modifications to their crystallized song over time. But in general, the adult song remains highly constant and stereotyped.
Timing of song learning stages
The onset and duration of sensory and sensorimotor learning phases can vary across different bird species. Here is an approximate timeline for a typical songbird such as a zebra finch:
| Age | Stage |
|---|---|
| 15-25 days | Sensory learning phase begins |
| 25-50 days | Subsong; sensorimotor learning begins |
| 50-80 days | Plastic song |
| 80-120 days | Crystallized song achieved |
In some species, crystallized song is achieved much later. For example, in canaries sensorimotor learning may continue until 9 months of age. Timing is also influenced by factors like nutrition, social interactions, and seasonal cycles.
Neural basis of song learning
Complex neural pathways underlie the different stages of song learning and production in birds. Key brain areas involved in vocal learning include:
- HVC – A sensorimotor integration area where auditory memories and motor programs for song converge. Drives temporal sequencing of song syllables.
- RA – Premotor cortex directly controlling respiratory and vocal muscles. Activity here translates into vocal output.
- LMAN – Generates vocal variability during plastic song. Allows exploration needed for trial-and-error learning.
- Area X – Part of a basal ganglia-thalamocortical loop regulating plasticity.
During sensory learning, tutored songs get encoded in the HVC. LMAN and Area X help generate variable subsong and plastic song as birds practice. Eventually, a direct motor pathway from HVC to RA emerges that allows crystallized song to be produced accurately without feedback variability.
Critical periods for song learning
Birds have sensitive time windows early in life during which they must learn song. If birds are not exposed to a tutor during these critical periods, they fail to develop normal adult song.
For sensory learning, there is a limited developmental critical period where young birds can form auditory memories of tutor songs. This critical period ends around 25-60 days of age depending on species. Tutor deprivation past this time results in inaccurate or absent song imitation later on.
There is also an age-limited critical period for sensorimotor learning. Young birds must practice vocalizing using auditory feedback for a limited time early in life, or they lose the ability to fully imitate song. This sensorimotor period may continue a bit longer, through 80-120 days of age approximately.
Some species such as canaries and starlings have an additional critical period for song plasticity late in the first year of life when they may still edit their crystallized song. But for most songbirds, plasticity declines rapidly once adult song emerges.
Role of social interactions
Social influences are vital for normal vocal development in young songbirds. Much evidence shows that young male birds selectively learn song from adult tutors, often their fathers. When multiple tutors are accessible, students may mimic certain song types more than others based on social dominance relationships and other factors affecting tutor choice.
Interacting with adult females also shapes song learning. Female songbirds provide feedback to young males as they practice singing. Different patterns of feedback from females can drive males to modify their songs during sensorimotor learning. For example, female canaries tend to respond more to males producing more complex songs, driving increased complexity over development.
Young birds also benefit from interacting with siblings. Sibling interactions tend to accelerate the process of song learning and maintenance of adult song.
Overall, the social song environment has a significant impact on the developmental trajectory of birdsong learning and the song repertoire that emerges.
Importance of auditory feedback
Auditory feedback is essential during the sensorimotor phase of birdsong learning. Young birds use their own vocalizations plus the auditory responses of other birds to shape their vocal output and match it to the stored song memory.
Experiments blocking auditory feedback during sensorimotor learning consistently show disrupted vocal development. For example, deafening birds or raising them in soundproof boxes can lead to highly inaccurate and variable song. Interfering with auditory feedback in adult birds usually does not alter crystallized song due to the decreased plasticity of this stage.
Interestingly, female adult zebra finches who cannot sing will start producing male-like song if they are deafened. This shows auditory feedback normally inhibits such vocalizations but is necessary for suppressing them once song circuitry develops.
Parallels with human speech acquisition
There are notable parallels between birdsong learning and the way human infants acquire speech and language:
- Both involve sensory and sensorimotor phases of learning. Auditory templates precede vocal production.
- Critical or sensitive periods affect learning outcomes if language/song exposure is delayed.
- Social influences shape vocal development in both birds and humans.
- Auditory feedback guides learning in both birds and babies as they practice vocalizing.
- Vocalizations transition from variable babbling to crystallized words/song.
Studying the different stages of birdsong learning gives insight into general principles of how the brain acquires complex learned behaviors, including human speech and language acquisition.
Conclusion
Song learning in songbirds occurs in two main phases: an early sensory phase when songs are memorized, followed by a sensorimotor phase when birds practice singing. Key stages of sensorimotor learning include subsong, plastic song, and finally crystallized adult song. Timing varies across species but there are critical periods when birds must hear song or practice vocalizing to learn properly. Social interactions, especially with adult tutors, are vital for guiding the song learning process. Auditory feedback also plays a crucial role. Understanding how birds learn song provides insight into speech learning capabilities that are shared across species.
