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)