Above: To other birds, this male broad-tailed hummingbird’s magenta throat feathers likely appear as an ultraviolet+purple combination colour. During the experiment, researchers found that the hummingbirds quickly learned that nonspectrum LEDs indicated a reward. Results are published in the journal Proceedings of the National Academy of Sciences. Photo: David Inouye


WHILE HUMANS HAVE three colour cone types in the retina, sensitive to red, green and blue light, birds have a fourth colour cone that can detect ultraviolet light. For a new study, a Princeton-led research team trained wild hummingbirds to perform a series of experiments that revealed that these tiny birds also see combination colours such as ultraviolet+green and ultraviolet+red.

“Humans are colour-blind compared to birds and many other animals,” said Mary Caswell Stoddard, the lead researcher. “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.” 

The team was particularly interested in the “non-spectral” colour combinations. Humans are able to detect just one non-spectral colour – purple. Birds can theoretically see up to five combinations: purple, ultraviolet+red, ultraviolet+
green, ultraviolet+yellow and ultraviolet+purple.

Wild broad-tailed hummingbirds (Selasphorus platycercus) were chosen to test the theory because they learn colour/reward associations quickly and with little training. Two feeders were set up; one containing pure water, the other with sugar water. Next to each were LED tubes programmed to display a broad range of colours, including non-spectral colours like ultraviolet+green. Over the course of several hours, wild hummingbirds learned to visit the feeder with the rewarding colour.

The results revealed that hummingbirds can see a variety of non-spectral colours, including purple, ultraviolet+green, ultraviolet+red and ultraviolet+yellow, and indicate that non-spectral colour perception is vital for signalling and foraging. 

Finally, the research team analysed a data set of 3,315 feather and plant colours. They discovered that birds probably perceive many of these colours as non-spectral, while humans do not.

“Tetrachromacy – having four cone types – evolved in early vertebrates,” said Ms Stoddard. She continued: “This 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 non-spectral colours is not just a feat of hummingbirds, but a widespread feature of animal colour vision.”

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