Anders Odeen and Olle Hastad of Uppsala University conducted experiments showing that what birds see isn't what humans see. They note that many studies about factors influencing avian sexual selection incorrectly assume birds see what we see.

"The results mean that many studies on sexual selection may need to be re-evaluated," said Odeen.

The researchers used mathematical models of the retina in birds and humans. The models used information on differences in the color-sensitive cone cells of the eye to determine how the richness and brightness of colors are perceived by birds and humans.

Birds' eyes, the scientists discovered, have four types of color-sensitive cones, while those of humans have three. Birds can also perceive ultraviolet light, whereas humans cannot.

The models were used to evaluate how birds and humans perceive different visual arrays set up by the experimenters. The researchers said their results show colors are perceived differently by birds and humans more than 39 percent of the time.

The study appears in the journal American Naturalist.