Figure 1: Time period of Sensory and Sensorimotor periods in two bird species; Zebra Finches (A) and White-crowned Sparrows (B). While both have each period, the sparrows' periods, like the hummingbirds', are very distinct from each other. (Mooney, R. (2009). Neural mechanisms for learned birdsong. Learning & Memory, 16(11), 655-669.)


Critical Period for Juveniles:

The critical period for vocal learning for hummingbirds occurs when the birds are juveniles, as it does for songbirds and parrots. Male hummingbirds are exposed to the songs of adult hummingbirds and later, develop songs of their own from the songs that they heard. Within the critical period, there are two phases, sensory learning and sensorimotor learning, that are both necessary for the juvineniles to develop their own songs. In sensory learning, a juvenile listens to and memorizes an adults song, and in the sensorimotor learning phase, the juvenile bird relies on auditory feedback to nearly match the song they sing to the one they listened to and memorized. This ultimately leads to the song become "crystallized," which is when the song the juvenile sings will become highly stereotyped and no longer dependent on the auditory feedback of another bird.

Phase 1: Sensory learning:

The sensory learning portion of vocal learning appears to be a form of imprinting, where the juvenile birds are able to memorize another song in a very short amount of time - sometimes only minutes - and there is a short window of time in which the juveniles can learn the songs. Juvenile birds have also been shown to have preferences in the songs that they learn, choosing those of their own species over another species' songs. Additionally, because sensory learning and sensorimotor learning occur with a separation of a few months between them, sensory learning results in a long-term memory, as most other imprinted memories and behaviors.

Phase 2: Sensorimotor:

In the second phase of vocal learning, the sensorimotor phase, the juvenile begins to produce its own song, which by using the memorized song of the adult and auditory feedback, eventually is able to reproduce the song of the adult. This phase occurs over much more time than sensory learning, a number of weeks, and involves three main parts. The first, subsong, is when the juvenile bird sings for the first time, producing rambling and "babbling" vocalizations that are not at all similar to the memorized song. Plastic songs, the second part of the sensorimotor phase, involve the juvenile beginning to vocalize with sounds that are found in the memorized song. As the juvenile practices their plastic song over a matter of weeks, they slowly decrease the variation found in their song and include many of the same components of the memorized song, leading to the final crystallized song of adulthood. These crystallized songs may still undergo changes over the birds' lifetime, something that is more common in certain species of hummingbird's than others (see Adaptive Value), but there is never as much variation to the crystallized song the adult birds sing as there is in the juvenile's plastic song.