Stages of song development
Normal song development proceeds through a series of stages. In the first, the young male memorizes the songs of one or more adult birds. In many species, males are most sensitive to memorize songs in the first few months of life, the so-called "sensitive phase." Actual vocal production begins during or soon after the sensitive phase when the male begins subsong. Subsong has been compared to babbling in human infants. The function is still not clear, but only song birds go through this stage; birds that do not learn to sing do not produce subsong. Here is a seven second long segment of subsong performed by a 240 day old white-crowned sparrow. Click on the sound spectrogram to hear it. In the laboratory, hand-reared birds perform subsong throughout their first autumn and winter.
As days lengthen in the late winter and early spring, subsong gives way to plastic song, in which the first evidence of imitations of tutors appears in the male's singing. Shown below is a 9 second long segment of plastic songs by a 260 day old sparrow (the 5-10 sec long quiet intervals between successive songs have been shortened to 2 secs here) containing three different songs. The three tutors that the male memorized during the sensitive phase are shown above, and are connected by arrows to the young male's imitations. Note that in plastic song, imitations are often incomplete. Also, note that the last song is a "hybrid" song composed of parts of two different tutor songs. This male is "overproducing," he sings more imitations than eventually appear as his final crystallized song. Click on each of the three tutors and the segment of plastic song to hear them, and compare the young male's imitations to his tutors.
In the wild, male song birds engage in matched counter-singing, in which territory neighbors sing similar song types back and forth to one another. We can simulate this in the laboratory by playing one song type (Tutor 1 above) that matches one of the song types a young male is practicing. Here is the same male, 35 days later singing his single crystallized song type. The first song is the tape playback, 3 seconds later the male responds with his rendition of the matching song type. He has stopped singing his imitations of Tutors 2 & 3, and will retain his imitation of Tutor 1 as his crystallized song. This is an example of selective attrition of songs from the overproduced plastic song repertoire, guided by hearing the song of another bird. This selective loss of songs may lead to vocal dialects, in which neighboring males sing very similar songs that differ from the songs at other locations.
