Better colour vision helps birds to find food, dazzle mates, escape predators and navigate diverse terrain. They are far superior to humans at detecting differences between two similar colours.
"Humans are colour-blind compared with birds and many other animals," said Mary Caswell Stoddard, an assistant professor in the Princeton University Department of Ecology and Evolutionary Biology.
Humans are trichromatic, which means they have three types of colour-sensitive cones in their eyes—attuned to red, green and blue light—but birds have a fourth type, sensitive to ultraviolet light. So birds are tetrachromatic.
"Not only does having a fourth colour cone type extend the range of bird-visible colours into the UV, it potentially allows birds to perceive combination colours like ultraviolet + green and ultraviolet + red—but this has been hard to test," said Stoddard.
To investigate how birds perceive their colourful world, Stoddard and her research team established a new field system for exploring bird colour vision in a natural setting. Working at the Rocky Mountain Biological Laboratory (RMBL) in Gothic, Colorado, the researchers trained wild broad-tailed hummingbirds (Selasphorus platycercus) to participate in colour vision experiments.
"Most detailed perceptual experiments on birds are performed in the lab, but we risk missing the bigger picture of how birds really use colour vision in their daily lives," Stoddard said.
"Hummingbirds are perfect for studying colour vision in the wild. These sugar fiends have evolved to respond to flower colours that advertise a nectar reward, so they can learn colour associations rapidly and with little training."
Stoddard's team was particularly interested in "nonspectral" colour combinations, which involve hues from widely separated parts of the colour spectrum, as opposed to blends of neighboring colours like teal (blue-green) or yellow (green-red). For humans, purple is the clearest example of a nonspectral colour. Technically, purple is not in the rainbow: it arises when our blue (short-wave) and red (long-wave) cones are stimulated, but not green (medium-wave) cones.
While humans have just one nonspectral colour, purple, birds can theoretically see up to five: purple, ultraviolet + red, ultraviolet + green, ultraviolet + yellow and ultraviolet + purple.
Stoddard and her colleagues designed a series of experiments to test whether hummingbirds can see these nonspectral colours. Their results appeared on June 15 in the Proceedings of the National Academy of Sciences.
The research team, which included scientists from Princeton, the University of British Columbia (UBC), Harvard University, University of Maryland and RMBL, performed outdoor experiments each summer for three years. First, they built a pair of custom "bird vision" LED tubes programmed to display a broad range of colours, including nonspectral colours like ultraviolet + green. Next they performed experiments in an alpine meadow frequently visited by local broad-tailed hummingbirds, which breed at the high-altitude site.
Each morning, the researchers rose before dawn and set up two feeders: one containing sugar water and the other plain water. Beside each feeder, they placed an LED tube. The tube beside the sugar water emitted one colour, while the one next to the plain water emitted a different colour. The researchers periodically swapped the positions of the rewarding and unrewarding tubes, so the birds could not simply use location to pinpoint a sweet treat. They also performed control experiments to ensure that the tiny birds were not using smell or another inadvertent cue to find the reward. Over the course of several hours, wild hummingbirds learned to visit the rewarding colour. Using this setup, the researchers recorded over 6,000 feeder visits in a series of 19 experiments.
The experiments revealed that hummingbirds can see a variety of nonspectral colours, including purple, ultraviolet + green, ultraviolet + red and ultraviolet + yellow. For example, hummingbirds readily distinguished ultraviolet + green from pure ultraviolet or pure green, and they discriminated between two different mixtures of ultraviolet + red light—one redder, one less so.
"It was amazing to watch," said Harold Eyster, a UBC Ph.D. student and a co-author of the study. "The ultraviolet + green light and green light looked identical to us, but the hummingbirds kept correctly choosing the ultraviolet + green light associated with sugar water. Our experiments enabled us to get a sneak peek into what the world looks like to a hummingbird."
Even though hummingbirds can perceive nonspectral colours, appreciating how these colours appear to birds can be difficult. "It is impossible to really know how the birds perceive these colours. Is ultraviolet + red a mix of those colours, or an entirely new colour? We can only speculate," said Ben Hogan, a postdoctoral research associate at Princeton and a co-author of the study.
"To imagine an extra dimension of colour vision—that is the thrill and challenge of studying how avian perception works," said Stoddard. "Fortunately, the hummingbirds reveal that they can see things we cannot."
"The colours that we see in the fields of wildflowers at our study site, the wildflower capital of Colorado, are stunning to us, but just imagine what those flowers look like to birds with that extra sensory dimension," said co-author David Inouye, who is affiliated with the University of Maryland and RMBL.
Finally, the research team analysed a data set of 3,315 feather and plant colours. They discovered that birds likely perceive many of these colours as nonspectral, while humans do not. That said, the researchers emphasise that nonspectral colours are probably not particularly special relative to other colours. The wide variety of nonspectral colours available to birds is the result of their ancient four colour-cone visual system.
"Tetrachromacy—having four colour cone types—evolved in early vertebrates," said Stoddard. "This colour vision system is the norm for birds, many fish and reptiles, and it almost certainly existed in dinosaurs. We think the ability to perceive many nonspectral colours is not just a feat of hummingbirds but a widespread feature of animal colour vision."