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Caleb Arnold loves birds, especially hummingbirds. The 7-year-old boy knows a lot about his favorite animal. "They pollinate (授粉)plants, and they're pretty and small, " he says. But there's one thing Caleb didn't know: hummingbirds see a wider range of colors than we do.
It makes sense that Caleb didn't know this. Few people did for sure, until June 15, 2020, when researchers published a report in Proceedings of the National Academy of Sciences. It confirmed the fact that scientists have long suspected. Birds are experiencing a more colorful world that is above our own. A team of scientists conducted a three-year experiment to find out if hummingbirds are able to tell apart colors that look the same to humans.
To understand the experiment, it's important to know how color vision works. There are three types of tiny color-sensitive cones (锥细胞)in the human eye. Birds, and some other animals, have a fourth. This means they can also see ultraviolet (紫外线的) colors. The ultraviolet cone type allows birds to see combination colors that humans can't, like ultraviolet green and ultraviolet red. Biologists studying birds have supposed that birds can see these, but it's been challenging to test this idea.
That's exactly what the team did. They put two bird feeders in the wild. One contained ordinary water. The other contained sugar water. Both feeders held a specially designed LED light stick—sort of a bird-vision light tube that changed colors. If we are shown the ultraviolet green tube and the green tube, they'd both just look green to us. But the hummingbirds saw the difference. They could even tell apart two shades of ultraviolet red. The scientists observed that the birds were able to use the colors of the lights to find the sweet water.
Caleb is glad that scientists want to better understand birds. "Good," he said, after hearing the news. "They're part of nature."