Pigeons concurrently categorize photographs at both basic and superordinate levels.

We studied categorization in pigeons, using carefully controlled photographs. Within daily sessions, 4 pigeons had to classify each of 32 photographs into either its proper basic-level category (cars, chairs, flowers, or people; four-key forced choice procedure) or its proper superordinate-level category (natural or artificial; two-key forced choice procedure). The pigeons successfully classified the same stimuli at both levels. Overall, the pigeons learned the basic discrimination more quickly than the superordinate discrimination, but this difference was reliable only for artificial stimuli (cars and chairs), not for natural stimuli (flowers and people). The pigeons also exhibited reliable discrimination transfer to novel photographs, attesting to the open-endedness of these basic and superordinate categories.