We humans use our vision for many things, but it's limited because it relies on the primary colors.
Some other animals, like birds, can see on the ultraviolet spectrum. A new camera developed by researchers at Lund University in Sweden lets us have a sense of how birds see the world.
Humans see in the visible spectrum between ultraviolet and red light. When light hits a surface, some of it gets absorbed and some is reflected. That reflected light enters our eyes where, after traveling through a couple of different parts of the eye, the light is essentially translated into colors by photoreceptor cells called cones. Most people have around 6 million cones, and each cone is attuned to a different wavelength of color.
So when you see a lemon, your eyes take in red and green wavelengths from the reflected light of the fruit. The various color-driven cones send that signal to your brain, which processes the number and strength of cones activated. With that information, your brain perceives that the color is yellow.
Birds also see primary colors, but they have additional cones that allow them to register ultraviolet light as well. We didn't know about this until the 1970s when researchers discovered, by accident, that pigeons could see ultraviolet (UV) light. It turns out that some feathers even reflect UV light. So, the colors that birds see are more varied than what humans see.
As to what this would look like, researchers weren't sure. "We can't imagine," Auburn University ornithologist Geoffrey Hill told The National Wildlife Federation in 2012 about a bird's vision.
Except now we can.
A bird's eye view of reality
To see the world how birds see it, researchers from Lund University developed a special camera that attempted to mimic the vision of birds. Designing the camera relied on calculations about birds' cones, the sensitivities of those cones and oils in the birds' eyes that help them discern different shades of colors better than humans can. The result was a camera with a rotating wheel of six filters.
Researchers captured 173 sets of six photographs — one through each filter — of different habitats, ranging from Sweden to Australia to rainforests.
Their "avian-vision multispectral camera" gave the researchers what they believe to be fresh insights into how birds navigate their habitats.
"We have discovered something that is probably very important for birds, and we continue to reveal how reality also appears to other animals," Dan-Eric Nilsson, a biology professor at Lund, said in a statement issued by the university.
Nilsson and his co-researcher Cynthia Tedore found that birds likely see the upper sides of leaves — the top of a forest canopy — in lighter shades of UV light, while the underside of the leaves is very dark. Where humans see a green mass either way, birds can discern where they are relative to the canopy simply by how their eyes interpret the UV light. This may help them navigate dense foliage and find food.
Of course, the camera isn't a true representation of how birds see reality, but it may be pretty close. Nilsson and Tedore conclude that their camera may provide a way to understand better "the evolution of vision and color patterns in natural habitats."
Tedore and Nilsson published their work in the journal Nature Communications.