Sex differences in brain and behavior are widespread across vertebrates. Birds exhibit remarkable examples of these types of parallels between structure and function. For example, only male zebra finches sing, and the brain areas and muscles controlling the learning and production of these vocalizations are greatly enhanced in males compared to females. These sex differences are permanently established in development. Some songbirds, unlike zebra finches, change their songs seasonally. In a number of these species, the brain regions exhibit changes in neuron loss and incorporation across these periods. The mechanisms involved in these types of sexual differentiation and adult plasticity are described-they likely involve both steroid hormones and genetic (protein) factors. The strength of the relationships between morphology and behavior, as well as many other factors, has made birds outstanding models for the investigation of numerous functions. These include the mechanisms regulating vocal learning, auditory perception, neurogenesis, and cell survival. The lessons learned have broad implications for health-related processes and basic biological principles. Table of Contents: Introduction / Song and Song Learning / Other Social/Reproductive Behaviors / Conclusions and Ideas for Future Directions