Relative colour cues improve colour constancy in birds.

A ripe strawberry looks red to our eyes in sunlight and in the green light of a forest, although the spectrum of light reflected from its surface differs dramatically. This is caused by two effects: colour constancy and our ability to learn relative colour cues - the ripe strawberry remains relatively 'redder' than an unripe green strawberry. While colour constancy - the ability to recognize colours in shifted illumination - has been studied in many animals, the use of relative colour cues is investigated more rarely. In a previous study on chickens, we measured how large a shift in illumination their colour constancy mechanisms tolerate without reliable relative colour cues. Here, we show that chickens remain colour constant over larger illumination shifts, if they can use such relative colour cues. As relative colour cues are readily available in natural environments, we suggest that their use contributes strongly to colour constancy performance in nature.