On the other hand, birds are tetrachromatic and they
have an additional UV receptor compared to humans,
making colour perception in birds more complex than in
mammals (Bowmaker, 1977). Further, they have oil
droplets that can act as filters that increase the complexity
of colour perception. Birds can see, in theory, twice the
number of colours compared with trichromats (O¨ deen and
Ha°stad, 2003). It seems that birds do not have an innate
preference for the red colour, although most of them have
their greatest spectral sensitivity and hue discrimination
towards the long wavelength end of the spectrum (Stiles,
1981). Experiments with hummingbirds have shown that
they learn to associate a range of colours with rewards and
that this behaviour can be modified (Proctor et al., 19