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The chaffinch studied in classic song-learning experiments is Fringilla coelebs, a songbird. Its male song combines inherited organization with details acquired through learning.

In his 1958 experiments, W. H. Thorpe compared wild birds, juveniles isolated after time in the wild, and birds hand-reared in auditory isolation from the fifth day of life. The early-isolated birds produced much simpler songs; birds isolated only in their first autumn retained nearly normal patterns. Groups of early-isolated birds could develop elaborate but abnormal shared songs by hearing and copying one another.^[ref]

A playback apparatus called the Song Tutor presented artificial patterns assembled from chaffinch notes. Birds learned some patterns when their tonal qualities were sufficiently similar to normal song. These results distinguish inherited dispositions from the auditory experience needed for local song details. Learning was also concentrated in a sensitive period, principally within the first thirteen months in these experiments.^[ref]

A critical period in neural development is a bounded interval in which particular experiences have a strong role in establishing a specified function or circuit organization. Its timing must be stated for a species and a measured outcome. A sensitive period emphasizes heightened susceptibility without necessarily asserting that every later change is impossible.

A classic example comes from monocular deprivation in cats. P. B. Dews and colleagues closed one eye at different ages and for different durations, then compared vision through the two eyes when the animals were at least a year old. Starting deprivation at one month offered little protection against lasting impairment; starting at two months reduced the impairment, and starting at four months or later produced no lasting deficit detectable in their behavioural tests. The age comparison revealed a developmental window rather than merely a short-lived effect of having an eye closed.^[ref]

The design separates age at disruption, duration of disruption, and the function tested. A change in visual acuity and a change in the responses of visual cortex neurons need not have identical recovery profiles. The existence of a critical period for one visual measure does not establish a single deadline for the entire brain, or the absence of adult neuroplasticity.^[ref]

A sensitive period is an interval of development during which a particular kind of experience has an unusually strong effect on subsequent behaviour or nervous-system organization. It is specific to a function and experimental outcome: susceptibility of visual acuity and susceptibility of a recorded neuron's response need not have identical schedules. Such timing is one aspect of neuroplasticity.

A revealing experiment varied both the age at which a kitten's eyelids were sutured shut on one side and the duration of closure. The investigators later compared vision through the two eyes using graded behavioural tasks. Starting deprivation at different ages produced different lasting effects, and longer early deprivation affected more demanding tasks. This separates the timing of experience from merely its total absence.^[ref]

The same paper reported some improvement when subsequent experience forced use of the previously deprived eye, including in one older animal. A sensitive period can therefore coexist with later improvement. Its experimental meaning depends on the measured behaviour, the timing and duration of deprivation, and the opportunity for recovery.^[ref]

A songbird, or oscine, is a bird of Passeri, a major branch of Passeriformes. The name denotes evolutionary membership, rather than a judgment about whether a particular call is melodious. The chaffinch is one familiar example. Relationships among these birds are studied using DNA and phylogenetic trees, alongside their anatomy and behavior.^[ref]

Songbirds provide particularly clear experiments on learning. In W. H. Thorpe's chaffinch experiments, birds raised without normal song exposure developed simpler songs, whereas birds with early experience could retain much more normal patterns. An apparatus called the Song Tutor replayed constructed sequences of notes, letting the investigator change what a juvenile heard rather than merely observe wild adults. Some artificial patterns were learned, provided their sound characteristics were sufficiently appropriate.^[ref]

The comparison separates three questions: what vocal patterns a bird can physically produce, what experience supplies, and when that experience is effective. A sensitive period means that the timing of exposure matters. These results establish a role for experience in the tested species; they do not imply that every songbird learns the same repertoire at the same age.