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| Costa Rican hummingbird, from Ask a Biologist |
Humans have only three color receptors, but birds are tetrachromats, with four color receptors. In addition to birds, many reptiles, fish, insects, spiders, shrimp, and other invertebrates can see colors beyond our range of awareness. The scientists estimate that as much as 35 percent of the color experience of birds includes colors we can't even imagine, not just additional colors, but combinations of colors such as ultraviolet plus green.
Some birds that look plain to our eyes have patterns that can only be revealed by translating those extraspectral colors into visible light. According to NatGeo:
"This extra level of discernment might also have been a trait of dinosaurs, which are thought to have sported colorful feathers. Mammals evolved as nocturnal beings that did not need to see the rich hues of the daylight world, so most—like your pet dog and cat—are dichromatic, and have only blue and green cones. People evolved a third cone (red), possibly because early primates developed an appetite for ripening fruits."
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More online resources
Science Friday Podcast: A Bird's Eye View of Color
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