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" (lor 2) items. In further tests, other new stimuli were introduced that consisted not only of these training numerosities, but also ofthe intervening ones (3, 4, and 5). Variationsin 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.

Two experiments are described in which pigeons were trained in a simultaneous conditioning procedure to discriminate small arrays of dots that differed in numerosity. The birds successfully learned to choose the array of each pair that contained fewer dots when these choices were reinforced and choices of the array with more dots led to timeout. For the majority of numerosity values tested, discrimination performance for a fixed S+ value was better when the numerical difference between S+ and S-values was larger rather than smaller. This effect was seen in the first experiment when the numerical difference value was shifted between training trials and novel test trials. In the second experiment, too, performance level depended on the size of the numerosity difference when the birds were concurrently trained with two difference values that varied across trials within sessions. However, discrimination accuracy was influenced secondarily by variations in the density, or interdot spacing, of the stimulus arrays. In order to explain the latter finding, it is suggested that a tendency to "scan" a lowdensity array incompletely might alter the probability of accepting it as the smaller numerosity (8+) stimulus. This would increase error rates with S-arrays in which the dots are more widely spaced.

After responding to each element in varying, successive numerosity displays, pigeons (Columba livia) had to choose, out of an array of symbols, the symbol designated to correspond to the preceding number of elements. After extensive training, 5 pigeons responded with significant accuracy to the numerosities 1 to 4, and 2 pigeons to the numerosities 1 to 5. Several tests showed that feedback tones accompanying element pecks, the familiarity of element configurations, and the shape of the elements were not crucial to this performance. One test, however, indicated that the number of pecks issued to the elements was important for numerosities above 2. An additional test confirmed that the birds chose the symbol that corresponded to a particular numerosity rather than the positions that the symbols had held during training.

Pigeons trained in a conditional discrimination procedure to respond to a visual array made a left or right choice, depending on which of two numbers of elements (i.e., anchor numerosities) the array contained. They were then tested with novel arrays at these anchor numerosities, as well as at interpolated and extrapolated numerosities. Various control conditions showed that the birds' discrimination p performance was primarily based on stimulus numerosity, and not on other factors, such as brightness or area. Results from a series of tests, spanning a wide range of numerosities, conformed to scalar p principles. Psychometric functions showed superposition, indicating that Weber's law applies to numerosity discrimination. The subjective midpoint between anchor values was at the geometric mean. V Variability about this bisection point increased in proportion to the numerical value of the mean.

Using operant conditioning methods, the pigeon's wavelength discrimination abilities were assessed in two experiments to generate discrimination functions. Both these functions showed three minima at 460, 530 and 595 nm. In the second wavelength discrimination experiment, extending measurements into the UV spectral region, pigeons also maintained good discrimination between wavelengths within the UV range tested. A fourth minimum was indicated at the lower end of the spectral range tested (365-385 nm). The results point to the complexity of the pigeon's chromatic system, which must be at least tetrachromatic, probably pentachromatic. eral times. A first function obtained by Hamilton and Coleman in 1933 had suggested that the pigeon's colour vision was similar to that of man. Later determinations (e.g., Blough, 1972; Riggs etal., 1972; Schneider, 1972; Wright, 1972a) have definitely corrected that erroneous early impression but they have not yet yielded sufficiently concordant functions. Moreover, none of the measurements has extended into the near-ultraviolet. This has become important since it has been demonstrated that pigeons are visually sensitive in this spectral region (see Kreithen, 1978; Wright, 1972b). We now report two experiments intended to improve this situation.