The social factor in song learning in american sparrows  (Page 6/9)

When do they learn?

Over the past few decades, the exact timing of song learning of whole songs (as opposed to song elements or parts of songs) has been a much-debated topic. No conclusive decisions have been reached among the scientific community on a proposed sensitive period for learning, specifically the strictness of this sensitive period (whether juveniles can only learn whole songs during this period) and the age that it occurs at. The overall theme, however, is that social interactions seem to play an important role in determining when songbirds learn songs. The rest of this section is a summary of recent discussions.

Marler (1970b) introduces the idea of a strict sensitive period for white-crowned sparrows in his discussion of the sensorimotor model of song learning. In this model, sparrows memorize songs between the ages of 20 and 50 days old; afterwards, they no longer memorize or learn any new songs. Instead, during their first spring, when they choose their territories for the first time, birds begin producing these songs from memory and compare them, via auditory feedback, to the songs of their neighbors. The birds then modify the songs stored in their memory to closely match the songs of their neighbors. In this model, the memorization/learning phase is called the sensory phase, while the reproduction phase/modifying phase is called the sensorimotor phase. The songs that are first produced during the sensorimotor phase are called plastic songs, because they are still subject to change or crystallization (Marler 1970b; see [link] ). The action-based model of song learning also indicates a sensitive period. In this model, a juvenile bird learns a large repertoire of songs and produces all of them during the plastic song developmental stage (Marler&Nelson 1993). When the bird meets its neighbor, it only selects the songs that most closely match those of its neighbors to keep.

Baptista and Petrinovich (1984) provided the first evidence against the strict 50-day sensitive period. They found that juvenile white-crowned sparrows can learn songs even though tutoring did not start until the juveniles were at least 50 days old, as long as social interaction between student and tutor is involved. In Marler (1970b)’s experiments, the juveniles were only taught by tape-recorded songs (no social interaction). Therefore, Baptista and Petrinovich conclude that using live tutors is a more effective way to teach juveniles, allowing them to learn songs after they reach 50 days of age. Juveniles, which in nature are tutored by live birds, can therefore be expected to be able to learn songs past 50 days of age as well.

Baptista and Petrinovich (1984)’s results were also a topic of debate. Cunningham and Baker (1983, described in Baptista&Petrinovich 1986) argue that the experiments performed by Baptista&Petrinovich simply shifted the sensitive phase. They claim that by not exposing juveniles to acoustic stimuli during their first 50 days of life, the sensitive phase for song learning is delayed. Then, when Baptista and Petrinovich reintroduced the live tutor stimuli after age of 50 days, the sensitive phase reopened. In response, Petrinovich and Baptista (1987) performed an experiment in which juveniles birds received song stimulation during their first 50 days of life and found that they were still able to learn after they became 50 days old. Nelson and Marler (1994) show just a bit of concession to Petrinovich and Baptista’s findings, arguing that not all birds can learn songs past their sensitive period of 50 days, and no bird can learn past 100 days old.