My favourite blue tit fact is that they are sexually dichromatic under UV light only. The male crown is brighter in UV and females prefer males with brighter crowns. Mere humans can't tell the difference.
I read a passage in a book growing up, I can't remember what book it was, but it was about how crows are only black to humans, they can see different colors than we can so they're very colorful to each other, covered in all sorts of brilliant colors.
Another fun fact: bees can see utraviolet, and flowers are "designed" for bees, so many of the patterns are invisible to humans, but visible under UV photography:
> It turns out that crows lack UV-reflective patches, and that the sexes really do look the same, plumage-wise. However, the research in Yorzinski’s lab unearthed subtle changes that indicate age: On the sides, back and even under the tail, feathers changed in hue, both in the human visual range and in the UV or violet range as the birds reached the age of 3.
While seeing the same wavelength range, human tetrachromats (4 types instead of 3 RGB receptor cones in retina) recognize 100M colors instead of the standard 1M.
>The common parakeet or budgie has some interesting UV features. There is a small UV reflective patch on the chin, which appears dark violet to humans. Also, unlike most yellow birds, the yellow feathers on the head of the parakeet strongly absorb ultraviolet light.
If you want a nice take on what that would be like for humans - Greg Egan has an interesting short story called “Seventh Sight” about people bio hacking themselves to gain expanded spectral perception. It’s in his Instantiation Anthology.
Sure, ultraviolet photoreceptors are cool. You know what would be cooler? What if birds, and their dinosaur ancestors, had evolved infrared phototransmitters that can lase? The world would be a different place. Imagine how upon hearing the distinctive call of the Red-Eyed Vireo animals and people would scramble for cover lest they become its next charred meal. On the other hand, self-roasting chickens would be a godsend for the industry.
Thank goodness birds are not real or this might have been a thing.
A fact related to something in the ocean, cephalopods have photoreceptors that are oriented neatly with nerves in the back, as you would expect. But mammals have the nerves coming out the front, in the way of the path of light! [1]
So, cephalopods have sensors that match a modern camera that's "back illuminated", with metal layer/routing out of the way, and humans have a old type "front" illuminated sensor [2].
The cavitation bubbles mantis shrimps create when they strike? We use that same technology to perform direct field manipulation in eye “surgery”. I put it in quotes because I’m not sure if that’s an appropriate word anymore! There is no mechanical slicing! We just place the energy where it needs to be, and thousands of cavitations reshape the eye!
As long as we are already on a tangent about rods, cones, and visual perception: one of my favorite fact is that the occasional genetic mutation that creates color blindness in men sometimes leaves a woman with "tetrachromacy". Four sets of rods and cones which increases color perception from some one-million colors to one-hundred million colors.
There was a neat story on the San Diego local news some years ago about a woman artist who was a tetrachromat. Fascinating interview. This woman said that the downside of perceiving additional colors is that something like the cereal aisle in a common supermarket could be a total sensory overload. But she of course said also that she appreciated the extra things she could see. :)
Comes from this rather brilliantly titled paper: Blue tits are ultraviolet tits https://pmc.ncbi.nlm.nih.gov/articles/PMC1688906/
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