Hummingbirds see colours humans can only imagine

Thanks to a fourth retinal colour cone that can detect ultraviolet light, hummingbirds see an array of combination colours involving UV that humans are blind to, according to new research by Princeton and UBC zoologists.

“Humans are colour-blind compared to birds and many other animals,” said Mary Caswell Stoddard, an assistant professor at Princeton University. Humans have three types of colour-sensitive cones in their eyes—attuned to red, blue and green light — but birds have a fourth type, sensitive to ultraviolet light. “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.”

So Stoddard and her research team established a field system for exploring bird vision in a natural setting at the Rocky Mountain Biological Laboratory (RMBL) in Colorado.

“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—including UBC PhD student Harold Eyster—were particularly interested in nonspectral colour combinations from widely separated parts of the spectrum, as opposed to blends of neighboring colours like teal (blue-green) or yellow (green-red).

“Our experiments enabled us to get a sneak peek into what the world looks like to a hummingbird,” said Eyster, a co-author of the study outlining the findings in the Proceedings of the National Academy of Sciences.

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 perceive just one kind of nonspectral colour — purple — birds can theoretically see up to five: purple, ultraviolet+red, ultraviolet+green, ultraviolet+yellow and ultraviolet+purple.

The research team from Princeton, UBC, Harvard, the University of Maryland and RMBL built a pair of custom ‘bird vision’ LED tubes that display a broad range of colours, including nonspectral. Next they performed experiments in an alpine meadow frequently visited by local broad-tailed hummingbirds.

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 rewarding and unrewarding tubes, so the birds could not simply use location to pinpoint a sweet treat. Over the course of several hours, wild hummingbirds learned to visit the rewarding colour.

Hummingbirds readily distinguished ultraviolet+green both from pure ultraviolet and from pure green, and they discriminated between two different mixtures of ultraviolet+red light — one redder, one less so.

"It was amazing to watch,” said Eyster. “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."

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.

"The colours that we see in the fields of wildflowers at our study site — the wildflower capital of Colourado — are stunning to us, but just imagine what those flowers look like to birds with that extra sensory dimension," said co-author David Inouye, with the University of Maryland and RMBL.

Finally, the research team analyzed 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 emphasize 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.