Hummingbirds can see an array of colors invisible to humans

This male broad-tailed hummingbird has magenta throat feathers that are likely perceived by birds as an ultraviolet+purple combination color.

(CNN)When hummingbirds make decisions regarding food, evading predators or choosing a mate, they're influenced by the diverse colors they can see that are invisible to human eyes, according to a new study.

These are known as nonspectral colors, or hues that come from largely separate parts of the color spectrum. Humans can see one nonspectral color, which is purple.
But it isn't part of the rainbow. Instead, we see purple when the short-wave blue and long-wave red cones in our eyes are stimulated, but not the third green medium-wave cone.
    Birds, however, have a fourth cone that can detect ultraviolet light. Their four-color cone vision is referred to as tetrachromatic.
      In a new study, researchers set up a field experiment to test how wild broad-tailed hummingbirds reacted to these colors.
      "Even though biologists have long assumed that birds can discriminate a variety of diverse nonspectral colors, our results confirm that this is indeed the case for hummingbirds," said Mary Caswell Stoddard, study author and assistant professor in the Princeton University department of ecology and evolutionary biology, in an email. "Our results are consistent with the idea that birds have tetrachromatic (four color cone) vision and can see a vast range of colors we humans can only imagine."
      The study published Monday in the journal Proceedings of the National Academy of Sciences.
        The researchers worked at the Rocky Mountain Biological Laboratory in Gothic, Colorado, and set up outdoor experiments and trained the hummingbirds to use them. These particular hummingbirds like to breed at a high-altitude alpine meadow that was used in the experiment. The experiments took place during the summer and over the course of three years.

        A discriminating eye

        They used a custom LED device to display two different colors on circular surfaces next to hummingbird feeders. One feeder would contain a reward, like sugar water, while the other contained just plain water.
        Before dawn, the researchers would set out one feeder containing each with an LED tube next to it. Each emitted a different color. The researchers regularly changed the positions of the colored tubes so the hummingbirds wouldn't associate one color with a reward. Each time, in just a matter of hours, the hummingbirds learned which color was associated with a reward.
        The LED devices displayed a range of colors, including nonspectral colors like ultraviolet+green, which is how the researchers refer to this combination of light components that the hummingbirds can see. When the researchers looked at green versus ultraviolet+green, the colors appeared the same to them.
        But the hummingbirds were able to distinguish the nonspectral colors, like purple, ultraviolet+green, ultraviolet+red and ultraviolet+yellow. They could even tell the difference between two different mixes of ultraviolet+red, or tell them apart from their components, like pure ultraviolet or pure red.
        "Personally, it was thrilling to watch hummingbirds learn to discriminate two different colors that appeared identical to us," Stoddard said. "Hummingbirds are smart, inquisitive birds — they learned new color associations very quickly. This makes sense because they evolved to respond to flower colors that advertise sweet nectar rewards."
        These birds may have more friends than you