When hummingbirds make decisions regarding food, evading predators or choosing a mate, they're influenced by the diverse colours they can see that are invisible to human eyes, according to a new study.

These are known as nonspectral colours, or hues that come from largely separate parts of the colour spectrum. Humans can see one nonspectral colour, which is purple.

But it isn't part of the rainbow. Instead, we see purple when the short-wave blue and long-wave red cones in our eyes are stimulated, but not the third green medium-wave cone.

Birds, however, have a fourth cone that can detect ultraviolet light. Their four-colour cone vision is referred to as tetrachromatic.

In a new study, researchers set up a field experiment to test how wild broad-tailed hummingbirds reacted to these colours.

"Even though biologists have long assumed that birds can discriminate a variety of diverse nonspectral colours, our results confirm that this is indeed the case for hummingbirds," said Mary Caswell Stoddard, study author and assistant professor in the Princeton University department of ecology and evolutionary biology, in an email. "Our results are consistent with the idea that birds have tetrachromatic (four colour cone) vision and can see a vast range of colours we humans can only imagine."

The study published Monday in the journal Proceedings of the National Academy of Sciences.

The researchers worked at the Rocky Mountain Biological Laboratory in Gothic, Colorado, and set up outdoor experiments and trained the hummingbirds to use them. These particular hummingbirds like to breed at a high-altitude alpine meadow that was used in the experiment. The experiments took place during the summer and over the course of three years.

A discriminating eye

They used a custom LED device to display two different colours on circular surfaces next to hummingbird feeders. One feeder would contain a reward, like sugar water, while the other contained just plain water.

Before dawn, the researchers would set out one feeder containing each with an LED tube next to it. Each emitted a different colour. The researchers regularly changed the positions of the coloured tubes so the hummingbirds wouldn't associate one colour with a reward. Each time, in just a matter of hours, the hummingbirds learned which colour was associated with a reward.

The LED devices displayed a range of colours, including nonspectral colours like ultraviolet+green, which is how the researchers refer to this combination of light components that the hummingbirds can see. When the researchers looked at green versus ultraviolet+green, the colours appeared the same to them.

But the hummingbirds were able to distinguish the nonspectral colours, like purple, ultraviolet+green, ultraviolet+red and ultraviolet+yellow. They could even tell the difference between two different mixes of ultraviolet+red, or tell them apart from their components, like pure ultraviolet or pure red.

"Personally, it was thrilling to watch hummingbirds learn to discriminate two different colours that appeared identical to us," Stoddard said. "Hummingbirds are smart, inquisitive birds — they learned new colour associations very quickly. This makes sense because they evolved to respond to flower colours that advertise sweet nectar rewards."

The researchers also analysed 3,315 feather and plant colours and found that 30% of feather colours and 35% of plant colours likely appear nonspectral to hummingbirds — even though humans can't make that distinction.

"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 David Inouye, study co-author and professor emeritus in the University of Maryland College Park's department of biology, in a statement.

Seeing in four colour cones

For the hummingbirds, this spectacular vision is normal and the colours reveal information about mates, predators and food, Stoddard said.

Understanding birds and how they sense and process the world can also help conservation efforts to protect them.

"We are now studying how hummingbirds use colour to find flowers and impress mates — and how these behaviours might be impacted by climate change," Stoddard said.

The researchers also want to test if hummingbirds learn at the same rate and if their learning is affected by blooming flowers. They recently studied courtship dives performed by the male broad-tailed hummingbirds, which show off their iridescent throat feathers while zooming past at 40 miles per hour, Stoddard said.

Humans may not have them, but these four cone types actually evolved in early vertebrates.

"This colour vision system is the norm for birds, many fish and reptiles, and it almost certainly existed in dinosaurs," Stoddard said. "We think the ability to perceive many nonspectral colours is not just a feat of hummingbirds but a widespread feature of animal colour vision. Did dinosaurs see nonspectral colours like UV+green and UV+red? We think so. How cool is that?"