Communication differs from other behaviors in the potential for an arbitrary relationship between a signal's function and the motor patterns used to produce it. Also, signals often incorporate motor patterns that have been co-opted from non-signal behaviors, resulting in a linkage between communication and other behaviors using the same functional systems. We examine possible evolutionary consequences of these attributes of communication, using birdsong as a focus. Syringeal function provides a case where co-adaptation with other behaviors is weak. By contrast, functional systems of the vocal tract have evolved in response to competing influences of divergent selective pressures. © 1992.