28 REVIEW Volume 33, No. 5 Lab Animal May 2004 Learning from Bird Brains: How the Study of Songbird Brains Revolutionized Neuroscience Bird watching has been popular for centuries, and songbirds have been favored subjects. During the past 30 years, songbirds have become popular laboratory animals as well. The expanded use of songbirds in the laboratory resulted from the discovery of the vocal control system (VCS), a series of well-defined brain nuclei that control song learning and production. The VCS rapidly became a favored model for studying how the brain controls behavior. A clearly delineated neural system with a well-defined behavioral function, it controls vocalizations, and to the best of our knowledge, nothing but vocalizations. This allowed researchers to relate changes in neural structure and/or function to changes in behavioral out- put. Research on the VCS rapidly demonstrated that these brain areas are exquis- itely sensitive to circulating hormone levels, both during development and in adulthood. Basic research on the hormonal control of singing behavior resulted in three unexpected findings about how hormones modulate brain function and behavior. Each of these discoveries fundamentally altered our understanding of how the brain functions. It is not clear if researchers working on songbirds were more open to these discoveries, but the study of bird brains definitely changed our view of how the brain functions. First, researchers discovered that in some species the anatomy of male and female brains differed so greatly that scientists could determine the bird’s sex by examining its brain with the naked eye 1 . These marked anatomical differences in the structure of the brain areas controlling singing behavior proved to be the result of exposure to particular hormones during critical periods in development. Until this research, many studies of neuroanatomy did not even mention the sex of the animals used. However, this discovery motivated researchers to search for such organizational effects of hormones on brain structures in other vertebrates, and researchers have documented many structural dimorphisms in brain anatomy in species ranging from frogs to humans 2 . Second, scientists discovered that the anatomy of the songbird brain changed seasonally as a result of shifts in circulating hormone levels, and that these changes were involved in activating alterations in singing behavior 3,4 . We now know that the modification of brain structure is a basic mechanism by which hormones alter an animal’s physiology and behavior, and researchers have found such activation- al hormone effects in every vertebrate examined. Some of these effects are quite rapid, occurring in a matter of hours rather than months 5 . Third, researchers discovered that one of the mechanisms used to alter brain anatomy in adult animals is neurogenesis (i.e., the birth of new neurons) 6 . These findings stimulated scientists to search for similar findings in other species, and once again they found that this was not merely an aberrant mechanism found only in songbirds. In fact, several studies have demonstrated adult neurogenesis in a variety of vertebrates, including humans 7,8 . Ultimately, this research reversed a central assumption in the field of neuroscience that had been held for more than a century, that neurogenesis does not occur in the adult mammalian brain 9 . Cheryl F. Harding, PhD During the past 30 years, songbirds have become a popular model for neuroscience research. The author reviews three funda- mental discoveries that have revolutionized the field and changed the way we under- stand the structure and function of the brain. Harding is in the Department of Psychology and Biopsychology Doctoral Program, Hunter College, City University of New York, 695 Park Ave., New York, NY 10021. Please address correspon- dence to the author at harding@genectr.hunter.cuny.edu.