How colourful are birds?

Lilac-breasted Roller, eveyone's safari favourite! Indigo & Violet

One of the best things about birding in the tropics is the sheer brilliance of many of our bird's plumage. After a day in the field you can easily think you've seen birds of every colour of the rainbow - and you're probably right! In a paper published last year by Mary Stoddard and Richard Prum (available free to all here) they demonstrate nicely that whilst they do cover the whole rainbow (and more besides), they still don't cover even the majority of the potential colours available to them - only about 26-30% of the potential options, it seems.

Red: Scarlet-chested Sunbirds use structural and pigmented colours

To understand how anyone can ever assess exactly how many colours are availalbe to birds, you need to start by understanding that many birds (though not all) actually see more colours than us - a lot can see ultraviolet light, as well as the usual combination or red, green and blue that we humans (and most other primates) see. (By contrast, most other mammals lack even our ability to see red, so they must live in a rather dull-looking world!) So by knowing the entire set of colours that can be generated from red, blue, green and ultraviolet the authors identified the potential range of colours available to birds, and they then sat down and looked at nearly 1000 (965 to be precise) sets of feathers and precicesly measured their colour, then plotted it in the red/green/blue/uv colour space. And they discovered that despite their efforts to find feathers covering as many different colour types as they could, they only found colours in about 1/3rd of the available space. In particular, birds seem to be missing a lot of the different options of green and purple. Now, there certainly are green and purple birds out there, but not all the possible forms of green, and not all the possible sorts of purple. (They're not particularly good at pure UV either - but that might in part reflect the author's inability to identify strongly UV feather groups in their initial search - we can't see it after all!)

Red-capped Robin-chats get orange from carotenoids in the diet

So why don't birds use all these possible colours? Well, there are probably two answers to this: on the one hand they can't really be expected to create every single colour going (especially given the pretty limited range of mechanisms they have available to create colours) and in particular getting close to pure colours of one extreme or the other is going to be extremely difficult to evolve, and on the other hand the birds might simply not like some other options, either they not attractive to mates, or they're particularly bad for avoiding predators for one reason or another (though the latter doesn't hold much water as an argument with me, as I'm sure many bright colours are essentially a deliberate handicap evolved by male birds to should how tough they are to females!).

Parrots, like these Yellow-collared Lovebirds have a unique yellow pigment

Yellow in a Speke's Weaver is from carotenoids in the diet

What's that about a limited number of options for how birds generate colours, I hear you ask? Well, yes, it's true. There are two basic mechanisms birds have for generating colours; they can use a pigment to reflect light, or they can change the structure of the feathers to generate refracted colours. The pigments are rather limited options, all birds can create melanin, which they can use to generate browns and blacks (it's the same pigment that makes black skin black, and my white skin a bit brown if I've been sitting in the sun). For many birds that evolved rather early on - like ostrichs, that it for pigments. But many other birds can also use another pigment group, the carotenoids to generate yellows, oranges, pinks and reds. Unlike melanin, the birds can't make these colours themselves, but must pick them up from their diet - it's well known that flamingos in zoos if they aren't fed correctly loose their pink colour and end up white (sad, but true). After that, there are only two other types of pigments that any birds use, and they're both really rather rare: the porphyrins that make Turacos green (and are also found in jacanas, bustards and pheasants), and the psittacofluvins that give parrots a yellow option. Four basic pigment sets, covering black, yellow, orange, green and red. Yes, you can mix them to produce some other options - but not many, and Stoddard and Prum's study showed they account for less than 1/3rd of all avian colours. Where birds really win (and despite the fact I'm trying to convince you they're not as colourful as they might be, they are more colourful that mammals or plants - even including flowers - so they do win!), is in the use of structural colours.

Blue on a woodland kingfisher - a structural colour

More structural blue, but the orange is from carotenoids - Superb Starling

Structrual colours are rather harder to understand than pigments - whilst pigments can be thought of as paints, there's nothing normal we have to generate structural coours. Essentially structural colours are formed by the birds leaving holes of precise shape in their growing feathers. These may be small, nearly perfectly round bubbles, or they can be long thin tunnels and we've recently learnt how they are formed in the growing feather - a simple property of chemical mixtures, it seems, though it involves mixtures of nanostructures showing once again that our highest technological advances with nanotechnology were developed by animals millions of years ago! Light passing through these bubbles and holes is split into different wavelengths in a similar way to light on the surface of soap bubbles can make rainbow colours. By being very careful about the size and shape of the bubbles and holes, specific colours can be chosen and they often have a metalic, glossy appearance (and look dark if light is coming from the wrong angle) - though further structural adjustment can ensure that the refracted light is completely scattered and appears similar from all angles and without the shiny/glssy appearance. Using these structural colours burds are able to create colours completely unavailable to plants and reach the colour spectrum other animals can't reach.

Irridescent green from structural colours: female Collared Sunbird

Again, a lot of birds mix structural colours and pigments to generate even more colours, but I still find it remarkable how many colours birds can produce given this rather restricted list of options. Why they bother is, of course, a whole other question that we'll have to think about another time! (PS this has really just be a great opportunity to post some of my favourite bird pics!)

Main reference:
Stoddard, M., & Prum, R. (2011). How colorful are birds? Evolution of the avian plumage color gamut Behavioral Ecology, 22 (5), 1042-1052 DOI: 10.1093/beheco/arr088