The study of white-crowned sparrow singing at the beginning of this chapter is a classic case of how genetics, physiology, and experience come together in the development and expression of behavior. Genetics endows the birds with a neural template that must be matched by experience during early development. That type of learning—imprinting—is limited to a sensitive or critical period by processes of developmental physiology. Months later, the onset of sexual maturation motivates the birds to sing, initiating a second phase of learning that requires the birds to use auditory experience to develop the motor patterns required for the full expression of species-specific song (Figure 52.7). At the end of this learning phase, the song becomes permanent, or crystalized. Thus the overall process of transmission of a complex species-specific behavior necessary for reproductive success is well adapted to preserving constancy of the behavior. But variation is an important component of evolution.

Variation in birdsong is evident in nature. There are subtle differences between the songs of any two males, including those in neighboring territories. Regionally, white-crowned sparrows have dialects, and a bird migrating into an area that has a different dialect can adopt that song pattern. There are even observations of white-crowned sparrows singing the songs of closely related species. In an interesting laboratory experiment, white-crowned sparrows were raised in isolation but exposed to the sight and song of a male of a closely related species while recordings of white-crowned sparrow song played on speakers. When those birds matured, they sang the song of the closely related species. The ability to incorporate variation into an individual bird’s song may relate to social interactions such as fitting into a regional norm, displaying individual qualities to a potential mate, and maintaining a bond between mates.

To investigate questions about the adaptive significance of variations in the male singing behavior, it is necessary to take female choice into consideration. Sarah Woolley and Allison Doupe at the University of California, San Francisco did that with zebra finches (Taeniopygia guttata), a species that has become a valuable model system for studying the neurobiology of birdsong. When male zebra finches sing in isolation, their song is variable (“undirected”), almost as if they are improvising and trying out new motifs. In the presence of females, however, they sing “directed” song (directed to a female) that is less variable. Woolley and Doupe conducted experiments to see which songs female zebra finches preferred (Investigating Life: Practice Makes Perfect). The females preferred directed song, and if mated, they preferred the directed song of their mate. Thus the ability to incorporate variations into the species-specific song may be important to establish individuality and influence mate choice, but the nature of the singing depends on the social context.

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