Spectacular bird's-eye view? Hummingbirds see diverse colors humans can
only imagine
Team trains wild hummingbirds to discriminate UV color combinations

Date:
June 15, 2020
Source:
Princeton University
Summary:
While humans have three color cones in the retina sensitive
to red, green and blue light, birds have a fourth color cone
that can detect ultraviolet light. A research team trained wild
hummingbirds to perform a series of experiments that revealed that
the tiny birds also see combination colors like ultraviolet+green
and ultraviolet+red.



FULL STORY ========================================================================== [Hummingbird feeding on | Credit: (c) Martha Marks / stock.adobe.com] Hummingbird feeding on flowers (stock image).

Credit: (c) Martha Marks / stock.adobe.com [Hummingbird feeding on |
Credit: (c) Martha Marks / stock.adobe.com] Hummingbird feeding on flowers (stock image).

Credit: (c) Martha Marks / stock.adobe.com Close To find food, dazzle
mates, escape predators and navigate diverse terrain, birds rely on
their excellent color vision.


========================================================================== "Humans are color-blind compared to birds and many other animals,"
said Mary Caswell Stoddard, an assistant professor in the Princeton
University Department of Ecology and Evolutionary Biology. Humans have
three types of color-sensitive cones in their eyes -- attuned to red,
green and blue light -- but birds have a fourth type, sensitive to
ultraviolet light. "Not only does having a fourth color cone type extend
the range of bird-visible colors into the UV, it potentially allows birds
to perceive combination colors like ultraviolet+green and ultraviolet+red
-- but this has been hard to test," said Stoddard.

To investigate how birds perceive their colorful world, Stoddard and her research team established a new field system for exploring bird color
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
color 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 color
vision in their daily lives," Stoddard said. "Hummingbirds are perfect
for studying color vision in the wild. These sugar fiends have evolved
to respond to flower colors that advertise a nectar reward, so they can
learn color associations rapidly and with little training." Stoddard's
team was particularly interested in "nonspectral" color combinations,
which involve hues from widely separated parts of the color spectrum, as opposed to blends of neighboring colors like teal (blue-green) or yellow (green-red). For humans, purple is the clearest example of a nonspectral
color. 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 color -- 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 colors. Their results
appear 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 colors, including nonspectral
colors 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 color,
while the one next to the plain water emitted a different color. 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 color. 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 colors, 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 colors, appreciating how these colors appear to birds can be difficult. "It
is impossible to really know how the birds perceive these colors. Is ultraviolet+red a mix of those colors, or an entirely new color? 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 color 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 colors 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 analyzed a data set of 3,315 feather and plant colors. They discovered that birds likely perceive many of these colors
as nonspectral, while humans do not. That said, the researchers emphasize
that nonspectral colors are probably not particularly special relative
to other colors. The wide variety of nonspectral colors available to
birds is the result of their ancient four color-cone visual system.

"Tetrachromacy -- having four color cone types -- evolved in early vertebrates," said Stoddard. "This color 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 colors
is not just a feat of hummingbirds but a widespread feature of animal
color vision."

========================================================================== Story Source: Materials provided by Princeton_University. Original written
by Liz Fuller- Wright. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Mary Caswell Stoddard, Harold N. Eyster, Benedict G. Hogan, Dylan H.

Morris, Edward R. Soucy and David W. Inouye. Wild
hummingbirds discriminate nonspectral colors. PNAS, 2020 DOI:
10.1073/pnas.1919377117 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200615155114.htm

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