Crow curiosities: can crows see UV?

Recently, a marvelous set of blue crow photos from Carl Bergstrom had the internet’s corvid fans doing a collective double take. In addressing what could be responsible for such spectacularly odd images, many people’s first instinct was to wonder if these photos might be revealing the hidden ultra-violet lives of crows. After all, as a group, passerines (aka songbirds, of which crows are part of) are well known for their abilities to express themselves and see beyond the visual spectrum available to people. But while, “can crows see in UV? Is their perception of the feathers adorning their flock mates different from our own?,” feel like simple enough questions, a google search after their answers results in an almost unprecedented silence from the otherwise vast body of crow knowledge that exists beyond your search bar. Sure, you can find the occasional popular science article that talks about the visual systems of birds and maybe includes a photo of a crow, but these articles never provide citations and most speak simply in generalizations about passerines, not about crows specifically. The reason for this knowledge gap is that while the visual systems of birds is generally well studied, there are over 10,000 species of birds and not all of them can be the darling of every field of research. So while crows take a disproportionate share of our scientific attention, relative to many other species, not much has actually been done on their visual systems; what does exist is spread out and sometimes hard to find. But this is a question that comes up time and time again so let’s take a moment to harness what has been done, and offer the best possible answers to these questions that science currently has to offer.

Before we get to the heart of our questions though, let’s take a beat to review the more technical aspects of vision, and why our visual experience of the world is different from our dogs’ or possibly crows’. Vertebrate eyes work fundamentally via the same 5 step process: Step 1) light enters eye through pupil, Step 2) the cornea bends the light that passes through the pupil, Step 3) the light then passes through the lens which focuses it on the retina, Step 4) rods and cones of retina detect light and color and, Step 5) cells in retina convert this into impulses which go to brain. But while the general process is conserved across most species, the details of each of these steps can vary in life altering ways. Crucial to this discussion is that fourth step that involves the rods (which are motion sensitive light detectors) and the cones (which are contrast sensitive color detectors). Depending on the classes of cones a species possess, an animal can be either dichromatic (most mammals), trichromatic (primates and marsupials), or tetrachromatic (birds and reptiles), which translates to different levels of color vision. ¹ While we are able to detect red, green and blue light, most birds have a fourth cone that allows them to more acutely detect short wavelength colors near the ultraviolet range. The ability to simply detect UV isn’t enough though (in fact humans are sensitive to UV light), you must also have the ability to transmit that part of the spectrum. While our eyes filter it out, rendering it invisible to us, birds have special oil droplets in their cones that allow for the passage of UV light, while limiting its damage.² Among birds, that 4th cone (called the short-wave sensitive 1 or SWS1) can be further divided into two variants: the violent-sensitive variant (VS birds) or the ultra-violet sensitive (UVS birds) variant. Without getting any more technical, suffice it to say that UVS birds have a much keener visual experience of the UV spectrum, relative to VS birds, though both can detect UV light.³

The function of this “enhanced” vision is many fold.⁴ For one, it allows for greater contrast of the environment, rendering what may look to our eyes as a flat wall of green vegetation, as a much more dynamic plane, enhancing a bird’s ability to fly through dense foliage. Like insects, UV sensitivity is also important among many types of nectarivorous (nectar drinking) and frugivorous (fruit-eating) birds. Many fruits, for example, are coated in a UV-reflecting waxy substance that helps advertise their availability to would be seed dispersing birds. And finally, descriptive UV patterns in feathers opens an entire world of visual signaling that is otherwise completely hidden from us. Given the ways we might image crows would benefit from exploiting any one of these possibilities, it makes sense that they would possess the kind of rich UV experience that many other birds are known for.

Which brings us, finally, to the rub. While it’s true that most passerines are what we call UVS birds, corvids, like flycatchers and most raptors, are VS birds, meaning their visual system is biased toward the violet-spectrum and they are not considered especially sensitive to UV light.^3,5 The low UV-detection abilities of corvids and many raptors, appears to offer a lifeline to smaller passerines, which exploit these visual differences in their plumage, allowing them to remain conspicuous to potential mates, while staying inconspicuous to their potential predators.⁶ Given this finding, we would expect crows not to, for example, show a great deal of UV detail in their feathers, and the research seems to bear this out. A study of large-billed crows found them to be so weakly iridescent, that the authors proposed their violet-blues hues may simply be an artifact of chance, and play no functional role.⁷ Likewise, unlike many other passerines, crows don’t seem to communicate aspects of their identify via secret codes in their feathers. A 2007 study, for example, confirmed that American crows, fish crows, and Chihuahuan ravens are sexually monochromatic from an avian visual perspective, meaning there’s no UV signaling of “male” or “female” hidden from us in their feathers.⁸ These birds were among only 14, of the 166 North American passerines sampled, for which this was true.

Despite these findings though, the role of UV in the lives of crows and other corvids hasn’t been rendered completely immaterial. When presented against high contrast backdrops (green foliage), fish crows are more adept at picking out UV reflecting berries than matte black Vaccinum berries. On the other hand, when both are presented in front of a backdrop that offers no contrasting advantage to the UV reflecting fruit (sandy backdrops) they pick out both berries equally.⁹ And while the UV spectrum may not be super useful to crows for coding information, that doesn’t mean the feathers of corvids don’t carry any weight. Common magpies, for example, convey all sorts of information from sex to age to territory status in their iridescent tail feathers.¹⁰ Taken together, these findings seems to suggest that there is a lot more to unpack with respect to the role of UV in the lives of corvids than, well, meets the eye, and species-specific studies may be necessary to fully parse the potential nuance.

In the mean time, while the errant photo of a blue crow may be eye catching, it’s probably not revealing an otherwise visually hidden secret, like that time a ghost showed up in the background of your vacation photo. Instead, blue crows are probably just an artifact of the photographer’s white balance gone awry in the golden hues of a fine day.

Literature cited

1. Bowmaker JK. 1998. Evolution of colour vision in vertebrates. Eye 12, 541–547
2. Lind O, Mitkus M, Olsson P, Kelber A. 2014 Ultraviolet vision in birds: the importance of transparent eye media. Proc. R. Soc. B 281: 20132209.
3. Ödeen A, Håstad O & Alström P. 2011. Evolution of ultraviolet vision in the largest avian radiation – the passerines. BMC Evol Biol 11: 313.
4. Withgott J. 2000. Taking a Bird’s-Eye View…in the UV: Recent studies reveal a surprising new picture of how birds see the world. BioScience 50: 854–859.
5. Brecht KF, Nieder A. 2020. Parting self from others: Individual and self-recognition in birds. Neuroscience & Biobehavioral Reviews 116: 99-108.
6. Håstad O, Victorsson J, Ödeen A. 2005. Differences in color vision make passerines less conspicuous in the eyes of their predators. Proceedings of the National Academy of Sciences 102: 6391-6394.
7. Lee E, Miyazaki J, Yoshioka S, Lee H, Sugita S. 2012. The weak iridescent feather color in the Jungle Crow Corvus macrorhynchos. Ornithol Sci 11: 59–64.
8. Muir DE. 2007. Avian Visual Perspective on Plumage Coloration Confirms Rarity of Sexually Monochromatic North American Passerines. The Auk 124: 155–161.
9. Schaefer HM, Levey DJ, Schaefer V, and Avery ML. 2006. The role of chromatic and achromatic signals for fruit detection in birds. Behavioral Ecology 17: 784-789
10. Nam HY, Lee S, Lee J, Choi C, and Choe JC. 2016. Multiple Structural Colors of the Plumage Reflect Age, Sex, and Territory Ownership in the Eurasian Magpie Pica pica. Acta Ornithologica 5: 83-92.