Birds have four types of single cones, which are sensitive to UV, short, medium, and long wavelengths, respectively. The birds were trained to obtain food from small containers printed with stimulus patterns.
Chicks learn the colours quickly and accurately.
The results indicate that all four cone types probably drive least three opponency mechanisms, as required for tetrachromatic colour vision. Birds trained to one coloured pattern and tested on a range of novel alternatives prefer the familiar hue and saturation, but are attracted by patterns with high luminance contrast.