Birds perceive colors in categories just like people do
Birds perceive colors in categories just like humans do: a recent study shows that zebra finches perceive red or orange, even when reality is a shade somewhere in between
When you look at a rainbow, do you see bands of specific colors, or do you see a continuum of colors? Most people see distinct bands of colors, despite the fact that the rainbow itself is a continuum across the entire visible spectrum of color. Why do we see what isn’t there?
Considering the amount of information reaching our sensory organs every second, it could easily become overwhelming if we did not mentally categorize it. This color categorization is why we see bands of colors when we look at a rainbow. But does color categorization originate from a cultural or linguistic basis, or does it stem from an inherent biological basis?
Linguists and psychologists have long debated whether language itself influences and shapes human perceptions, particularly when it comes to colors. Some researchers argue that color categories have a cultural or linguistic basis, and that these categories must be identified by their own names before people can accurately and quickly distinguish colors (ref).
But other researchers dispute this idea, arguing that color perception arises from a biological basis that does not depend upon either culture or linguistics. This comes from the observation that, regardless of language or culture, specific color names always cluster around the same hues, and also that human infants can differentiate between red, yellow, green, blue, and purple even before they learn the names of these colors (ref). It was that finding that provoked the original impetus for this study: if human infants can distinguish colors without knowing their names, then surely, birds can do it too?
Obviously, colors are an important visual signal for plants and animals, otherwise, they wouldn’t waste the energy creating, maintaining or displaying colors. For example, carotenoid-based colors are often used by birds as visual signals for selecting mates (ref). But individual birds vary in their ability to obtain and metabolize carotenoids, and this difference is how a female identifies a high-quality mate. For example, in zebra finches, Taeniopygia guttata, the carotenoid-based beak coloration in males ranges from pale orange to dark red, with females showing a clear preference for males with red beaks (ref). Male zebra finches use beak color as a visual signal, too: males with red beaks are higher up the social dominance hierarchy.
Previous research has established that birds respond to both ends of a carotenoid-based color continuum — orange versus red — but whether they perceive variation in hue as being continuous or if they exhibit categorical color perception is unknown.
How red is “red enough”?
To create a spectrum of colors from orange to red, sensory ecologist Eleanor Caves, a postdoctoral fellow at Duke University, and her collaborators selected eight Munsell colors previously used to describe the color of zebra finch beaks (i.e.; ref). These colors were approximately equidistant from each other within the spectrum that zebra finches are most sensitive to under ambient light. Using these colors, Dr. Caves and her colleagues created paper discs that either had one of the eight pre-selected colors on it (“solid”), or had two of the pre-selected colors, one on each half (“bicolored”) in different pair combinations. They then trained female zebra finches to flip over bicolored discs that had been placed over a well containing millet seeds, a favorite food reward.
Dr. Caves and her colleagues designed an elegant series of tests to learn whether the color pair on each bicolored disc affected the birds’ abilities to discern between the two colors. In experimental trials, they presented 26 female finches with a foraging grid with twelve wells, six of which were covered with discs: two solid discs for each of the two colors, and two bicolor discs comprising the same colors as the solid discs. Birds passed a trial if they flipped both bicolor discs before flipping any solid discs, indicating that they perceived the two colors on the bicolor disc as being different.
Dr. Caves and her colleagues found the boundary in the transition from orange to red hues, and saw that the birds were best able to discriminate between two colors located on opposite sides of this boundary line. Thus, the birds responded more strongly when the difference in color between the two halves of the bicolored disc were perceived to be clearly orange whilst the other was clearly red.
“What we’re showing is: he’s either red enough or not,” said the study’s senior author, Stephen Nowicki, a professor of biology at Duke University, in a press release.
Why are female zebra finches so fixated on red?
Red is an important color for many vertebrates because they cannot create their own carotenoids, yet they use it to signal quality. Thus, animals that are red, such as birds, must obtain their carotenoids from other sources, usually by consuming invertebrates or plants that do manufacture carotenoids.
Red is important because females use it to identify a high-quality potential mate. Previous studies indicate that female birds rely on red as a visual signal to indicate the quality of a male’s territory or his foraging abilities. Red color also positively influences a male’s social status. But equally important, one of the key enzymes that metabolize carotenoids into red coloration in birds is also an important enzyme in the immune pathway (ref), so a red beak (or red plumage) also indicates that a male bird has a robust immune system. Thus, a female zebra finch can quickly assess a male’s overall health and fitness simply by choosing a mate with a red beak.
Such a visual signal is referred to as an “honest signal” because it cannot be faked.
Why does categorial color perception matter?
Although the light-sensitive cells in a bird’s eyes can distinguish different wavelengths of light within the same color category, apparently, it is the brain itself that decodes and processes these visual signals into distinct categories.
“What hits the retina is not always what we see,” Dr. Caves explained.
It is possible that, by decoding color into discrete categories, the brain may be making it easier for the bird (or person) to deal with the vast amount of noisy, limited or ambiguous sensory input from the environment. Categorical perception may be one strategy that the brain evolved to reduce ambiguity.
“We’re taking in this barrage of information, and our brain is creating a reality that is not real,” Professor Nowicki explained. Categorizing visual stimuli is a strategy for making it less important to precisely interpret a stimulus when simply identifying its category is sufficient.
So far, birds are the only animal outside of primates, that show categorical color perception. The evolutionary distance between primates and birds suggests that categorical perception of colors is probably much more widespread, and comes from deeper biological roots, than originally thought. This raises important questions regarding what influences color perception; for example, might the intensity or spectral purity of a color influence how it is categorized?
Comparative questions are also interesting, particularly whether birds perceive the boundary between orange and red color categories the same way that people do? The answer to this question is not yet known, but these findings suggest that color categories were not arbitrarily created by human language or culture. Comparative questions will likely inspire a wider survey of categorical color perception amongst other animals and provide more complete insights into the evolution of color vision as well as neural perceptions of color.
Eleanor M. Caves, Patrick A. Green, Matthew N. Zipple, Susan Peters, Sönke Johnsen and Stephen Nowicki (2018). Categorical perception of colour signals in a songbird, Nature, 9(4):968–980 | doi:10.1038/s41586–018–0377–7
Debi Roberson, Ian Davies, and Jules Davidoff (2000). Color categories are not universal: Replications and new evidence from a stone-age culture, Journal of Experimental Psychology, 129(3):369–398 | doi:10.1037/0096–34220.127.116.119
Geoffrey E. Hill (1990). Female house finches prefer colourful males: sexual selection for a condition-dependent trait, Animal Behaviour, 40(3):563–572 | doi:10.1016/S0003–3472(05)80537–8
Mirre J.P. Simons and Simon Verhulst (2011). Zebra finch females prefer males with redder bills independent of song rate — a meta-analysis, Behavioral Ecology, 22(4):755–762 | doi:10.1093/beheco/arr043
T. R. Birkhead, F. Fletcher, and E. J. Pellatt (1998). Sexual selection in the zebra finch Taeniopygia guttata: condition, sex traits and immune capacity, Behavioral Ecology and Sociobiology, 44(3):179–191 | doi:10.1007/s002650050530
Originally published at Forbes on 9 August 2018.