A chimera is an organism made up of cells from two or more genetically distinct sources. While some chimeras (very rarely) arise naturally through the fertilization of an egg by more than one sperm cell, or from the very early fusion of fraternal twin embryos shortly after fertilization, the avian brain chimeras considered here are different because they are produced surgically by the substitution of presumptive nervous system cells between the early embryos of different bird species. The word “presumptive” is used because the transplanted cells have not yet definitively become nervous system cells at the time of transplantation. This is because the cell substitution takes place before major organ systems or blood vessels are formed in the embryo. The surgeries are guided by “fate maps” that identify which groups of embryonic cells give rise to particular parts of the developed brain. Avian brain chimeras are used in basic research examining the mechanisms responsible for the development of, and evolutionary changes in, neural circuits: complex groups of interconnected nerve cells located in many different parts of the brain that regulate particular perceptual, cognitive, and behavioral functions of organisms.