The pigeons learned to associate a specific icon for each of 16 basic categories which included "dog." The pigeons were shown a black-and-white photograph of a dog along with two icons -- one that represented the word dog and the other that was randomly selected. If the pigeon pecked on the right icon, it was rewarded with pellets of food. Over time, the pigeons learned the icon for "dog" was a category that included photographs of a variety of dog breeds, like those shown above. They made the same association for 15 other categories.