With pigeons we are striving to understand the perceptual and associative mechanisms of bird cognition, mainly in the visual domain. As birds are highly mobile animals, vision is of particular importance to them and the study of pigeons is thus relevant to our understanding of the more general mechanisms of visual cognition. Though devoid of language and presumably also of the associated higher cognitive capacities pigeons can learn, categorize and remember large numbers of diverse types of visual stimuli, some of them as complex as those encountered in ordinary human experience. Pigeons have, for example, shown able to deal with line drawings, photographs, real objects, and even holograms and movies. The main goal of our research is to investigate the cognitive strategies (including picture memorization, feature learning, prototype formation and the abstraction of concepts) that underlie pigeons' response behaviour in different types of categorization tasks, with categorization thus being understood as a flexible and adaptive mechanism of selecting from the numerous strategies available. Within this, we are particularly interested in exploring the parameters that determine which stimulus aspects control categorization.