Herbranson, W. T., Karas, E., & Hardin, G. (2017). Perception of angle in visual categorization by pigeons (Columba livia). Animal Behavior and Cognition, 4(3), 286-300. https://doi.org/10.26451/abc.04.03.07.2017

Pigeons are capable of learning to categorize stimuli based on visual features, and often reach levels of accuracy comparable with humans. Nevertheless, recent research has suggested that the cognitive processes behind categorization in pigeons and humans may not always be the same. Pigeons learned a categorization task in which they categorized either Shepard circles varying in size and orientation of a radial line (stimuli that are frequently used in research on human categorization), or moving dots varying in speed and direction of travel (stimuli that have been successfully used to study pigeon categorization). Even though categories were balanced so that the angles of orientation of Shepard circles matched the directions of travel for moving dots, birds failed to learn categories based on the former but not the latter. Results suggest that information about angle as a direction of travel may be more important for pigeons than information about angle of orientation.