Spectral stimuli form a physical continuum, which humans divide into discre
te non-overlapping regions or categories that are designated by colour name
s. Little is known about whether non-verbal animals form categories on stim
ulus continua, but work in psychology and artificial intelligence provides
models for stimulus generalization and categorization. We compare predictio
ns of such models to the way poultry chicks (Gallus gallus) generalize to n
ovel stimuli following appetitive training to either one or two colours. If
the two training colours arc (to human eyes) red and greenish-yellow or gr
een and blue, chicks prefer intermediates, i.e. orange rather than red or y
ellow and turquoise rather than green or blue. The level of preference. for
intermediate colours implies that the chicks interpolate between the train
ing stimuli. However, they do not extrapolate beyond the limits set by the
training stimuli, at least for red and yellow training colours. Similarly,
chicks trained to red and blue generalize to purple, but they do not genera
lize across grey after training to the complementary colours yellow and blu
e. These results are consistent with a modified version of a Bayesian model
of generalization from multiple examples that was proposed by Shepard and
show, similarities to human colour categorization.