Due to a phenomenon called categorical perception, zebra finches partition the range of hues from red to orange into two discrete categories, much like humans do.
For a small, reddish-beaked bird called the zebra finch, sexiness is color-coded. Males have beaks that range from light orange to dark red. But from a female's point of view, a male's colored bill may simply be hot, or not, new findings suggest. Due to a phenomenon called categorical perception, zebra finches partition the range of hues from red to orange into two discrete categories, much like humans do.
The finding comes from a Duke University experiment that tested the birds' ability to tell whether colors are the same or different.
Using different pairwise combinations of eight hues representing the range of male beak colors, the researchers showed 26 females a set of quarter-sized paper discs, some two-toned and some solid colored.
The birds learned that each time they flipped over a two-toned disc with their beak, they found a millet seed treat hidden underneath. If they flipped over a solid colored disc, they got nothing. Picking a particular disc first before the others was a sign that a bird perceived it as having two colors rather than one.
Some trials involved color pairs that were closer together on the color spectrum, while others involved pairs that were farther apart. Females had no difficulty discriminating the most dissimilar pairings. What was interesting was how they treated the various hues in between.
The findings suggest a threshold effect at work, a sharp perceptual boundary where orange turns to red.
The birds were much better at distinguishing two colors from opposite sides of the boundary than pairs from the same side, even when all pairs were all equally far apart on the color spectrum.
Previous research had shown that zebra finch females prefer red-beaked males to orange ones, because redness correlates with good health.
Categorical color perception in zebra finches isn't likely to be just the result of variation in how well the light-sensitive cells in the birds' eyes distinguish different wavelengths, the researchers say. It may be in their minds.