Pigeons' serial ordering of numerosity with visual arrays

Pigeons were trained in a conditional discrimination paradigm to differentiate successively presented visual arrays according to the relative number of their elements. Transfer tests with novel stimuli demonstrated that they discriminated the categories of ''many'' (6 or 7) from ''few'' (1 or 2) items. In further tests, other new stimuli were introduced that consisted not only of these training numerosities, but also of the intervening ones (3, 4, and 5). Variations in the birds' discrimination performance corresponded to the order of stimuli on a numerosity dimension. This serial ordering was maintained when other factors such as brightness, size, shape, area, and contour of the elements were systematically controlled across tests. Smaller numerosities were somewhat better discriminated than those at the higher end of this test range.