Pigeons are a favorite animal to study in the laboratory. Pigeons have excellent visual acuity, color vision, and visual memory, all of which rival or even surpass these abilities in highly visual primates. Birds are also an important taxonomic group to compare with mammals, the other major group of warm-blooded, highly mobile, visually oriented animals. Although a large portion of the avian central nervous system is devoted to visual processing, the bird brain is just a fraction of the size of our own. It is this unique combination of high visual competence and small size that makes birds so central to our understanding of the basic mechanisms of visual categorization.