Bird song is a set of complex, temporal sound patterns, usually produced by males during the breeding season. Bird calls, on the other hand, are shorter, simpler, and are produced by both males and females throughout the year. Unlike songs, which usually have a learned component, Calls are generally experience- independent. Bird songs impart information specific to the singing individual. Unfamiliar songs indicate the presence of a stranger, and he may be attacked. For this reason, strangers will usually wait and listen to the songs of his new environment before proceeding to sing on his own.
In experiments with White-crowned sparrows, Marler utilized a tape recorder as a tutor to teach birds their songs. For these birds, the critical period, in which they are especially sensitive to their environment, is 10-50 days after hatching. Marler recorded that after this time, young sparrows develop a subsong, which is an unstructured twittering. Next, a plastic song emerges which has components of the adult song, but is more variable. Finally after 200 days, the mature song becomes permanent. The timing of the critical period is specific to each species, but the general experiment gives us a model from which to understand song learning in birds. There appears to be a genetic template that allows the young birds to learn a conspecific song. Birds can learn the songs of other species, but preferentially learn the song of their own species.
Marler hypothesized that song dialects could be a mechanism of reproductive isolation, which results in adaptation to habitats and eventually, speciation. Dialects may serve to discriminate against strangers. If each habitat has a local variant, a conspecific new to the area would still sing like a stranger.