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Prehistoric Colour
Skip to the summary if you must.
I think we need to brighten things up a little after the last major topic, both literally and figuratively. We’re going to discuss the colour of animals we will never meet. There are occasions where the preservation of a fossil is so exquisite, we are able to identify the colour. For anyone growing up in the 80’s, where much older illustrations abounded that erred on the side of dull brown, grey, and green caution regardless of species, this seems too mind-blowing to be real. Nevertheless, if you see an illustration of a Microraptor that isn’t irridescent black or an Anchiornis whose head isn’t ginger, it’s either the picture that is older than the discovery of those pigments in the species, or it’s the artist that is not up to date with the latest science. Let’s be fair to the artist here: a miscoloured species is not as wrong as putting an Iguanodon thumb spike on its nose. There’s a fairly good chance that there were more than one breed of a species with differing colouration (as is the case with the mammoth) or even that the male of a species is differently coloured than the female (most species of bird). Even the age of a single individual can alter its colour (keep it under your hat). In millions of years’ time, some sentient species may find a perfectly preserved tabby cat and draw it tortoiseshell – thus being right by accident even though the scientific literature of the day insists they were tabby.

My cat trying to stop me reading Imaginative Realism

The distant future artist that guesses these markings correctly will be a very lucky being.

It’s possible but it’s less likely than sticking to the known colour for a species. It’s not playing it safe with the colouration.

I don’t want to spend too long discussing the colouration of species for which we know the colour. If you want more information on that subject, there’s a great article here. Instead, let’s talk about how we’re going to colour the less well-preserved and the undescribed – because there’s still a lot we can do to avoid being wrong. It is tempting at times to believe that, in nature, anything is possible: Look at the vividly bold colours of the Hyacinth McCaw or the Golden Pheasant; consider the metallic sheen of the Kingfisher and the Grackle. There really does seem to be no limit to nature’s creativity. Why then should we, nature’s illustrators, limit ours? Why couldn’t Thescelosaurus have been electric green like a parakeet with lavender polka dots? What’s wrong with hot pink and cinnamon striped Tyrannosaurs? Why couldn’t the woolly rhino have been patterned like a peacock’s tail?

The answer lies in how colour is applied in nature: just like a 16th Century Realist Painter, nature does great things with a very limited palette.

Skip Melanin discussion
Melanin
Bird colour starts with 2 main pigments: eumelanin and phaeomelanin. Eumelanin added to white skin or feathers will make a darker grey as the concentration is added until it eventually reaches black. Penguins, Auks, Microraptor, Crows, Archaeopteryx, and Terns are all predominantly coloured with Eumelanin.

Phaeomelanin is ginger in its purest concentration and blonde at its weakest but, when mixed with Eumelanin, accounts for all the shades of brown. You need only look at human hair to see the full gamut of possible colours achievable with just these two pigments (beware dye-assisted colours, choose your human wisely). If our hair is coloured by the same processes that colour feathers, why doesn’t our hair exhibit the same crazy patterns? Feathers are a complex structure that starts out life as a spherical skin growth. During the feather’s development, the melanin pigments are laid down in varying combinations, causing the wide range of feather patterns we see around us1. Hair, by contrast, is a (comparatively) simple affair. Melanin concentrations do vary over time but, due to hair’s one-directional growth, the best you’re going to get is a linear gradient. The best example I ever saw of this was an elderly woman in the Wimpy bar in Woolwich whose knee-length hair had never been cut beyond the necessary amount to avoid split ends: The tips were blonde; the roots were grey; and the hair transitioned smoothly through brown in-between.

One last point on the topic of melanin before we move on: heavy deposits of Eumelanin strengthen the feather, so black in a region of a bird or non-avian Dinosaur may well be the result of a selection bias for stronger feathers in that part of the body. Some gulls have black wing tips while no flightless bird does, for example. The gull needs the strength for flight, the cassowary does not.


Carotenoids
Skip carotenoid discussion
We’ve discussed the melanins and, for mammals, this seems to be the end of the road for integumentary colouration. For birds (and likely their feathered ancestors too) this is only the beginning. After melanin, the next major source of bird colour comes from the diet: Carotenoids are responsible for the red & yellow pigments present in bird feathers, feet, beaks, and eyes. Yellow pigment comes from foods like seeds, buds, insects and fruit; red comes from shellfish. Birds have, within themselves, the ability to turn yellow carotenoids red, which explains the Cardinal: a seed-eating bird that is bright red. I don’t know of any case of a bird that was able to turn red carotenoids yellow though, so be careful depicting a yellow Baryonyx (blonde is safer if you must). Carotenoid pigment can cover any range from yellow through orange to red but there’s a caveat: multiple carotenoid colours do not seem to coexist within a single feather. Go ahead: try and find a single feather which has a pattern of scarlet red and canary yellow. You will find reds and blondes within the same feather because melanins can lay down the pattern around the solid colour of the carotenoid.

The Implausible Feather

Your eye knows there’s something wrong before your cognitive brain has identified what it is.

Blue is almost always caused by the separation of white light by the structure of the feather, hence the often metallic appearance. You can tell if a feather is structurally blue by moving the angle of it in relation to yourself. Does the colour change? That’s a structural colour and not a pigment. The exception to this is the Turaco, which combines blue pigment with yellow carotenoids to produce green.2, 3


Gradient Colours
Skip gradient discussion
It’s important to identify what is causing the gradient. If the effect is caused by ever-thinning feathers of one colour overlapping another band of colour, then the gradient has a very distinctive look, with some little gaps diagonal to the direction of the feather where barbs haven’t been preened perfectly. This should not be a smooth gradient because of the fluffy and often-tousled nature of feathers. Nature can make smooth transitions like this in other media – the green to reddish-purple of the plant below, for example.

An example of green to red/purple colour gradients in the plant world.

This plant does not apply an overlap to achieve its green to red transition, so the gradient is smoother than in feathers.

There are some colour bands of feathers which appear to transition smoothly from one colour into another, with no regard for feather displacement. These ones, almost without fail, will follow the colour wheel around to achieve the transition. One band of colour overlapping another can be a gradient from any colour, to any other colour; one colour slowly transitioning into another must go through the classic spectrum of colours. The exception are the gradients toward desaturation – any melanin-based colour into grey. I’ve yet to see a purple to grey gradient in any bird, I suspect it falls outside the range possible with the feather but there are enough birds out there to concede that I may be proven wrong. I hope I am.


Miscellaneous “Rules” of Bird Colour
We’ve covered a lot in this post: what causes colour in birds and why it doesn’t apply to mammals; what patterns and colour combinations are common and which ones are more rare. I’m going to sum up here for the “TL:DR” crowd and add a few more points for you brave souls who read all the way to the end (there will also be some things the skippers won’t find here in the summary but you did in the text).

  • White to black through grey are the most common colours for birds and are caused by Eumelanin.
  • Tan to dark brown are the next most frequent and are caused by mixing Eumelanin with Phaeomelanin.
  • Yellow, oranges, and red are caused by dietary Carotenoids and have been placed in order of most frequent occurrence.
  • Blonde to ginger are achieved by pure concentration of Phaeomelanin – less common than the above colours but not unusual. You never seem to find pure ginger on bird feet.
  • Structural colours are rarest – those are the metallic-looking blues, greens, and purples.
  • Melanins can create patterns on feathers and rings on beaks but cannot create complex patterns on skin.
  • Carotenoids cannot create in-feather patterns.
  • Carotenoids tend to be deposited at the rump and tail, while Melanins are found mostly on the flight feathers and head.
  • In species where the male and female are coloured differently with Carotenoids, the male tends to be red, while the female is yellow-green. The reverse has not been observed.

I hope this post helps you as much as collating this information has helped me. I like to write posts that I myself would have liked to find. If I’m alone in finding it useful, then so be it – but I suspect I’m not.

References
1. Development and Evolutionary Origin of Feathers, Prum 1999.
2. Manual of Ornithology, Proctor & Lynch, 1993. Yale University Press. P91.
3. Bird Colouration, Hill, 2010, National Geographic.

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