An avian analogue to human artificial or synthetic grammar learning (Reber, 1967) was developed. Pigeons viewed horizontal strings of three to eight colored letters. These strings either conformed to Reber's artificial grammar or violated it in one or two locations. Pigeons categorized the letter strings as grammatical (left keypeck) or nongrammatical (right keypeck). Overall accuracy of categorization was above chance to both familiar training strings and to novel transfer strings, thereby satisfying a conventional criterion for learning an abstract concept. The results support a multiple mechanisms point of view according to which pigeons, like humans, learn both abstract concepts and specific strings, or specific parts of strings, in artificial grammar learning tasks.