Hummingbirds See Beyond The Rainbow
Conventional wisdom suggests hummingbirds really like the color red—it’s the reason many commercial hummingbird feeders are made to look like a kind of red blossom. But it turns out that two items that both look “red” to humans may look very different to a hummingbird. That’s because these birds can see colors that humans cannot.
Humans see colors through photoreceptors called cones, and we have three of them for red, green, and blue colors. But most birds, reptiles, and even some fish also have fourth cone that’s sensitive to UV light. That means they can see further into the spectrum than we can, and that they can see “non-spectral colors”—combinations of colors that aren’t directly adjacent on the rainbow, such as red+UV and green+UV.
Mary Caswell Stoddard, an assistant professor of ecology and evolutionary biology at Princeton, set out to study whether hummingbirds actually make use of that ability in their everyday lives. Her team of researchers developed special LED lights that could display specific blends of colors, and used them to label feeders visited by wild hummingbirds in Gothic, Colorado. They found that the birds were able to learn which of several different UV+ blends of light would lead them to a sugary reward, even when the researchers themselves could not see any difference in the labeling colors. Their research was published this week in the academic journal Proceedings of the National Academy of Sciences.
Stoddard joins SciFri producer Alexa Lim to talk about the research, the evolution of color vision, and what a world with additional colors might be like.
Mary Caswell “Cassie” Stoddard is an assistant professor of Ecology and Evolutionary Biology at Princeton University in Princeton, New Jersey.
IRA FLATOW: This is Science Friday. I’m Ira Flatow. Picture the colors of a rainbow– red, orange, yellow, so forth. We can see the rainbow colors by using three different color-sensing cones in our eyes. A dog or a cat has only two types of color-sensing cones, so their rainbow is different.
Most birds and reptiles, though, have four color-sensing cones, meaning they can see colors we can’t experience. Boy, are they lucky. Science Friday Producer Alexa Lim has the rest of the story.
ALEXA LIM: Remember in the movie Wizard of Oz when Dorothy is flying around in a sepia-colored tornado, but once she lands, she opens the door to the technicolor Emerald City, full of color? Hummingbirds see the world differently from humans, literally, because their eyes can see a wider range of color than we do. No special glasses needed.
Humans have three types of color-sensing cones for red, green, and blue light. But most birds have all those plus one more, a cone that lets them sense ultraviolet. That doesn’t just give them one extra color. They can see combinations of ultraviolet plus other colors, like ultraviolet plus green. Now, researchers have found that hummingbirds can use those color combinations we can’t see to distinguish and learn about the food sources they visit.
Mary Caswell Stoddard is an Assistant Professor of Ecology and Evolutionary Biology at Princeton and one of the authors of a study published this week in the journal Proceedings of the National Academy of Sciences.
Welcome back to Science Friday, Dr. Stoddard.
MARY CASWELL STODDARD: Thank you so much for having me. It’s great to be here.
ALEXA LIM: You say these birds are seeing non-spectral colors. What does that mean?
MARY CASWELL STODDARD: Well, that’s right. We were really interested in understanding how hummingbirds perceive color. And as you mentioned, birds have the ability to detect UV wavelengths, and that’s because they have a fourth color cone type that is very interesting for two reasons. The first is that it extends the spectrum of visible colors for birds. So that means that if you’re a hummingbird looking up at the rainbow, you’ll see all the colors we can see– red, orange, yellow, green, blue, indigo, violet– plus ultraviolet.
But the second thing that the UV cone does, in theory, is provide birds with an extra dimension of color perception relative to humans. And that’s because this UV cone type should allow birds to see a vast range of combination colors, like UV green and UV red. Some of these combination colors are considered non-spectral. And when we say non-spectral, we mean a combination color that arises when the color cones that are stimulated are stimulated by light from widely separated parts of the color spectrum.
So for humans, purple is the clearest example of a non-spectral color. Purple’s technically not in the rainbow, but it arises when our blue and red cones, but not the green cones, are stimulated. And what’s really cool about birds is that, in theory, they have many different kinds of non-spectral color. So we humans have just purple, but birds can theoretically see purple, UV red, UV green, UV yellow, and UV purple.
ALEXA LIM: Wow. And you said it’s more than just an extended color palette. It’s an extra dimension?
MARY CASWELL STODDARD: Yes. One way we like to think about this is in terms of a color space. So we tend to represent human colors in a color space that looks like a triangle. Each vertex represents one of our three color cone types.
But for birds, we often represent the colors they can see in a tetrahedron. And each of its four vertices represents one of the bird four color cone types. And so we predict that birds can see colors throughout this tetrahedral color space.
ALEXA LIM: Wow. Can you give me an example of what kind of information is coded in this extra color sense then? What are they using it for?
MARY CASWELL STODDARD: Well, it turns out that these kinds of combination colors are really common in the environments of birds. So we were able to analyze a large data set of flower and plumage, feather colors, and we could predict that about 30% to 35% of those colors would be perceived as non-spectral by birds. But a much smaller fraction of those colors would appear as non-spectral to humans.
So we think that these colors are– they’re out there. They matter. They’re ecologically relevant for these birds. They’re using them to find food. They’re sometimes using them to attract mates.
For example, the male broad-tailed hummingbirds perform spectacular courtship dives. And as a male zips over the head of a female, his magenta iridescent throat feathers are on full display. And we estimate that that magenta color would appear to birds as a non-spectral UV purple.
We don’t think that non-spectral colors are particularly special relative to other colors in the environment for birds. They are just part and parcel of a tetrachromatic color vision system. So these colors are interesting to us because we see them as very different, and they haven’t received much formal research attention. But what matters to a bird when it’s looking at a color isn’t whether it stimulates this cone or that cone, it’s what does this color reveal about mates, predators, or food.
ALEXA LIM: So colors are– they’re kind of arbitrary. I mean, it’s not like there’s, like, a definitive color. It’s just kind of how they’re being interpreted in our brain?
MARY CASWELL STODDARD: That’s absolutely right. It is a sensation that arises in the brain and, in that sense, they’re subjective. It’s an interaction between the outside external world and wavelengths of light that are being reflected and how our eyes and brains are interpreting those wavelengths of light.
ALEXA LIM: Wow. That’s pretty cool. So I want to– I also want to talk about how you tested this, because I know you used wild hummingbirds. That doesn’t sound like the easiest setup.
MARY CASWELL STODDARD: Definitely not.
ALEXA LIM: So what did you do to test this?
MARY CASWELL STODDARD: Well, we trained hummingbirds to participate in color vision experiments. Each morning, we got up really early, and we set up two feeders. One feeder contained sugar water, and the other contained just plain water.
Next to each feeder, we placed a special LED light tube. And we designed these tubes to display a whole range of bird-visible colors, like UV green and UV red. The two beside the sugar water emitted one color while the tube next to the plain water emitted a different color. And we frequently swapped the positions of the rewarding and unrewarding tubes so the birds couldn’t just memorize the location of the reward. We also wanted to make sure that the birds weren’t using smell or some other cue to find the reward, so we performed a series of control experiments.
Our feeders were set up in the middle of a meadow, and this meadow was visited by dozens of hummingbirds, so we could observe whether the birds tended to visit the rewarded or the unrewarded feeder when they came back in search of a sugary snack. So over the course of several hours if the birds could distinguish between the two colors we were testing, we saw that they tended to visit the rewarded feeder. And using this setup, we were able to show that hummingbirds can see a variety of these nine spectral colors, such as UV red, UV green, purple, and UV yellow.
ALEXA LIM: All the hummingbird feeders we put out there are usually red. For some reason, we think hummingbirds love red. Do they actually prefer that color?
MARY CASWELL STODDARD: They do love red, but they don’t have an innate preference for it. They have learned to associate red with highly rewarding flowers or sometimes highly rewarding artificial feeders. Most bird feeders are red. And in our experiments, we show that hummingbirds could indeed learn to associate a red light with a reward, but they quickly lost this preference when we rewarded another color instead. So they can rapidly unlearn this preference for red if something else is rewarded.
ALEXA LIM: OK, so they’re pretty trainable. We’re training them.
MARY CASWELL STODDARD: They really are. We said that we trained them to participate in color vision experiments, but really, they required very little training. They’re ideal test subjects for a study like this.
ALEXA LIM: Right. So humans have three cones. Birds have four cones. What happened to our cone? Why did it drop out of the evolutionary tree? Or did birds evolve more?
MARY CASWELL STODDARD: Well, that’s a great question. So birds actually didn’t evolve an extra cone. They have retained the ancient vertebrate color vision system, which had these four different color cone types. So many fish, reptiles, even dinosaurs are, or were, tetrachromatic with these four color cone types.
It turns out that mammals lost two of the color cone types early in their evolutionary history when they were mostly nocturnal. So we humans enjoy our very modest color vision only because old-world primates re-evolved a third color cone type by mutating one of the existing cone types. So the point here is not that birds are superior– it’s that humans are so basic.
ALEXA LIM: You said it. Yeah, right. Well, I guess on that note, are there any animals that are less basic than us, that have more than four cone type?
MARY CASWELL STODDARD: Well, we will be considered extremely basic when we look to other animals in the animal kingdom. So the answer to that is yes. Probably the most famous in this respect are mantis shrimps, which have up to 12 different photoreceptors for color. Butterflies can have up to nine distinct color photoreceptors.
So there are certainly animals out there with really interesting color vision. I think we have a lot more work to do in our field to understand how these photoreceptors are wired and whether they all contribute to color vision in the way that we expect. But these are really big open questions in our field.
ALEXA LIM: There’s this entire communication system that’s out there that we’re not even attuned to. We can’t even tap into.
MARY CASWELL STODDARD: So that’s really true. I mean, these are the things I daydream about. What is this color experience really like for birds? Is it like a quantum leap in color experience, the same way that color TV is just so much better than black and white TV?
I mean, the truth is that we don’t know, and we cannot say. We don’t really know what a UV red color looks like to birds. Is it a mixture of those two colors? Or is it a sublimely new different color? We can only speculate. And I think that’s what makes studying bird perception both so tricky and so fun.
ALEXA LIM: Well, thanks so much for joining us.
MARY CASWELL STODDARD: Thank you so much for having me.
ALEXA LIM: Mary Caswell Stoddard is an Assistant Professor of Ecology and Evolutionary Biology at Princeton. For Science Friday, I’m Alexa Lim.