Blue Feathers – almost coherent scattering

A collection of Mountain Bluebirds at the local museum

This year, Stellar’s Jays have been common in my neighbourhood, it’s the only wild blue bird we get here. But, it has a secret, it’s feathers are pigmented to be black. They look blue because of an optical trick that occurs within the feather’s structure. And Stellar’s jays aren’t the only one using this trick, all the birds out there with blue feathers are doing the same thing.

In the late 1800s, naturalists used the recently discovered concept of Rayleigh scattering to explain why blue feathers were blue. Since tools to examine the nanostructure (structure in the order of a billionth of a meter) of a feather hadn’t been invented yet, naturalists assumed that a feather contained tiny transparent cells full of particles the right size to create Rayleigh scattering. Like the sky, blue light would be more efficiently scattered. As a result, to our eyes these feathers would appear blue.

Because Rayleigh scattering is incoherent, it produces the exact same colour irregardless of the observation direction. Since blue feathers in natural light don’t change colour depending on viewing direction, the assumption that their colour was formed through Rayleigh scattering seemed valid — until someone looked closer.

In the 1930’s, scientists examined a a non-iridescent blue feather under a directional light source. Colour variations were observed as the light source was moved – an iridescent characteristic that called into question the hypothesis of Rayleigh scattering making the feather blue.

By the 1940’s, a cool new gadget became available – the electron microscope. Now the internal nanostructure of blue feathers could be directly examined. On the first look, the internal feather structure appeared to contain randomly spaced objects. This meant scattered light would be incoherent resurrecting the hypothesis of Rayleigh scattering being responsible.

It took until the 1970’s for scientists to finally determined that the nanostructures were, in fact, not fully random. Instead they were a quasi-ordered matrix – not quite the perfect order of iridescence but not the full randomness required for Rayleigh scattering. Under natural light from all directions, like sunlight, these feathers appear to be the same colour from all directions. However when a directional light is shone on blue feathers the colour will change depending on the light direction.

A Blue Jay wing (I don’t have a close up of a Steller’s Jay)

Since the colour of a Steller’s Jay’s feather comes from its internal structure on a tiny scale, a damaged feather would lose its blue colour. The dark pigments in the feather, that act to help show off the blue, would make a damaged feather would look almost black. So if you are lucky enough to find a Steller’s Jay feather, take care of it to keep it blue

Thinking of Blue Skys

A blue sky behind some blue flowers

It’s rainy season on my island in the Pacific. The days are grey and not particularly inviting. We’ve been staying inside a lot, which can drive me a little bit stir crazy. So last weekend, when the sun came out we crammed in some time outdoors. With the mid-winter browns and greens that are typical around here (my island is not tropical), the blue sky was the most vibrant colour around.

Although we perceive blue blanketing the sky, in reality, the sky has no colour. Instead, the hue is created from the interaction between our atmosphere and the incoming sunlight. Our atmosphere is made up a bunch of different stuff —  nitrogen (78%) and oxygen (21%) with bits of dust, water vapour and some inert argon, among other things (some of which we’ve put there).

Water vapour and dust are the physically biggest components of the atmosphere, and are relatively large compared to the wavelengths of light. When light hits the water vapour and dust, it’s reflected in different directions, but the light remains white (an example is the clouds). So why does the sky appear blue?

Over time, all sorts of theories have surfaced to explain the blueness, which started heading the right direction with Goethe’s 1810 explanation: “If the darkness of infinite space is seen through atmospheric vapours illuminated by the daylight, the blue colour appears.” His theory said the sky’s colour comes from something within the atmosphere during the light of day — which is true, but vague.

About the same time a more scientific inquiry was being made into the nature of scattering light. John Tyndall showed in an 1869 lab experiment that the blue hues of the sky could be created when white light was scattered by tiny particles. A few years later in 1871, John William Strutt, aka Lord Rayleigh, was the first to describe the actual mechanism that makes the sky appear blue was a result of light interacting with gas molecules in the atmosphere.

These gas molecules are tiny compared to the wavelengths of light – several thousand times smaller. When light strikes one of these molecules, that molecule absorbs a specific wavelength (or colour) of the light’s energy and later re-emits the same colour in all directions; a process called Rayleigh scattering.

Most of the longer wavelengths of light pass through our atmosphere unaffected, resulting in the full spectrum of sunlight with a higher ratio of blue wavelengths from the scattering. For this extra blue light to make the sky appear a brilliant blue, a dark background is required. Fortunately, beyond our atmosphere is the blackness of outer space, which makes an ideal dark background. The combined effect of the extra blue light and the black of outer space results in a sky that appears blue.

If you shift your gaze towards the horizon, the brilliant blues give way to paler colours and perhaps even white. The light reaching you from near the horizon passes through much more atmosphere, so the scattered blue light is scattered again and again, reducing its intensity. Preferential scattering of blue light by our atmosphere occurs everywhere, not just above us. For example, light reflected from your hand to your eye is affected by this scattering, but the effect is so minuscule we can’t detect it. Over a longer distance, like to a range of distant mountains, there is enough atmosphere to superimpose a blueish haze on our view of the mountains.

Since the creation of a blue sky overhead is entirely depended on the preferred wavelengths the molecules in the atmosphere absorb — in our case, molecules in Earth’s atmosphere absorb energetic light (blues) at a much greater rate than less energetic light (reds). So the blue we see above us is an Earth thing, on another planet, the sky could look dramatically different. Check out the possibilities here.

the green butterfly


rain forest in Costa Rica

I have a thing for butterflies – or more specifically for their erratic flight patterns and metallic shades. Even the cabbage white butterflies living in my backyard (and letting their young eat my cabbage), have this hint of iridescence giving the white of their wings a hint of shine. Sadly, my temperate climate doesn’t offer the full spectrum of butterfly bling that’s out there (but does save me from spiders with jaws strong enough to break my skin (mostly)).

A recent post I read about fear and spiders in Costa Rica, got me thinking about my own trip to the country years ago. Jungles are exotic – at least to me since I’ve always lived at mid-latitudes. They’re both fascinating and frightening, I certainly wouldn’t want to be out there alone at night. This trip was one of the few opportunities I’ve had to wander through jungles (and I didn’t get eaten!).

Out hiking, I spotted an iridescent green butterfly just off the trail. The green was colour of a granny smith apple and glittered with every flap the butterfly took. After getting my camera ready, I followed the insect through the undergrowth (mildly foolish, I know). I got lucky and it let me get close.


The best shot I got of the green butterfly. If anyone can identify it, please let me know.

The iridescence in a butterfly’s wing (or dragonfly’s body, or a rooster’s tail, or even an oil slick on water), isn’t from a pigment. Instead an optical trick is required. In the butterflies case, the surface of the wing are covered in scales with a depth of one quarter the wavelength of blue (or green) light. Then the light reflected off the surface is augmented by the light reflected off the back surface giving the insect that fabulous iridescent effect.

I’ve written more on the physics behind iridescence here and here.

Some tidbits


Black amaranth growing in a random place.

Here are some tidbits of science-y garden bits I’ve come across in the last little while.

Blue Food
I grow blueberries and blackberries. My brassicas all have a nice blue-ish tint. Last year I grew blue tomatoes (not tasty enough to bother growing again) and this year I’m trying to grow blue popcorn. I have black amaranth from last year volunteering itself everywhere, which I’d argue fits into the same colour category as those above – which is really more purple than blue. So, Why are so few foods blue?

Along the same vein of blue, I stumbled across this berry eons ago – too bad it isn’t edible (it sure is pretty).

Space Grass
We’ve been actively converting grass to vegetable garden here. This spring, we doubled my veggie growing space by taking over most of the front lawn (the area in front of the food forest). It has the most sunlight of anywhere on my lot, so I’ve filled the space with beans, corn, amaranth, sunflowers and brassicas. What I don’t want is grass, so I was surprised to read about astronauts growing grass on the space station. Why not more food? Lettuce has been grown successfully up there. Or more flowers? There has already been zinnias in space.

Hot Peppers
One of the podcasts I listen to recently had an interview with one of my favourite authors – Mary Roach. The interview was about her most recent book (Grunt) which came out earlier this month, and I ordered. The book is about the science behind keeping soldiers alive, I’m hoping to start reading my copy this weekend. It turns out she was inspired to write Grunt after a research trip to study the science behind hot peppers. Here’s the article.

Although, I have no plans to weaponize my hot peppers, my plants are growing big and healthy. Hopefully I’ll get a bumper crop to turn into hot sauces.

some brassica appreciation

My spring bulbs have burst into bloom – grape hyacinths, daffodils and snow drops are all showing off their colours. And the unseasonably warm weather has coaxed the plum tree behind the back fence to bloom (which isn’t necessarily a good thing as I haven’t seen a single bee). As pretty as the flowers are this time of year I really appreciate the brassicas for both being pretty (in a more subdued way) and their tastiness. So here are some cabbage-family pictures.


A volunteer kale that looks nothing like the kale I originally planted.


Purple sprouting broccoli which I should harvest soon


A kale sporting one of my favourite colour combinations of purple and green


Morning dew on my last January King Cabbage – I had to zoom in as this cabbage is roughly the size of a golf ball.


Tatsoi in a cold-frame – which clearly I’ve neglected as it has gone to flower. The little yellow flowers are lovely though.


Let loose the puce


A squished flea may be the inspiration behind the colour puce

‘Tis the season to let loose the puce – and the rest of the shades on the pink-lilac continuum. An over abundance of these shades creates a cacophony of ugliness only equaled by the ‘toys for girls’ aisle of the local big-box-toy-store. Puce hides amongst the pink hearts and flowery sentiments created to sell cards and fancy underpants, but unlike bubble gum pink, puce has a bloody story behind it like magenta, tyrian purple and cochineal red (and surly we can all manage more than just one romantic day a year).

Puce’ brings thoughts of something putrid or rotten to my mind, the word sounds unpleasant to me. Like being up to ones elbows working in ground beef, perhaps to make a meatloaf, only to discover the meat is well past it’s expiry date. Or a colour to inflict on bridesmaids – which it often is (I didn’t do this to my bridesmaid).

Puce is a shade somewhere between a dark-purplish-brown and a dark-red and can even be a lighter red-purple. As described by Jude Stewart in her book Roy G. Biv, puce is “purplish brown reminiscent of raw chicken meat, prunes sweating in hot water, or the blood-filled French flea for which it is named.” Some say, it’s the colour of a blood-sated flea (however, no matter how much blood they eat, fleas stay the same colour). Perhaps, this colour comes from the bloody smear a squished flea makes, or is the colour of a flea’s blood-stained droppings.

Fashionable colours wax and wane over time – puce hit its high note at the court of the French queen Marie Antionette (1755-1793), perhaps because a flea’s hunger for blood was a metaphor dripping with sexual innuendo. This was also just after synthetic dyes were discovered by accident by a teenage chemist was attempting to make a synthetic quinine, the cure for malaria. Colours previously only available through complex and time-consuming manipulation of pigments requiring a dyer with the skills of an alchemist suddenly were available in large quantities. If you had the means, the shade of puce could grace your clothing, bedding, carpets, furniture, drapes and even the fabric covering your walls …. okay, maybe that’s puce overload.


This poppy could be described as puce

Shades along the pink-lilac continuum generally don’t make the list as anyones favourite colour – certainly not mine. I don’t hate these colours but only want to see it in small doses, like on a few poppies in a field, as too much pink is unsettling bringing to mind fairy princess castles combined with drunk-tanks and Pepto-bismol (okay there is a small minority of pink lovers out there, and if the colours from pink continuum works for you, embrace it).

Alternately, puce can be an intense green – a colour I would quite like.

Introducing the chicken-coop fly


I’ve decided to call these insects chicken-coop flies as their universe seems to be centered on the coop

While digging up photos for my golden buprestid beetle post from yesterday, I found a couple other great examples of iridescence from the yard. The metallic fly above and dragonfly below. Eons ago when I started this blog I wrote about how iridescent colours are formed – here is part 1 and part 2 of my explanation.


Another shiny garden visitor


Backyard Jewels


A golden buprestid – pretty enough to be a jewel

Nature’s best visual trick is iridescence, which can transform an ordinary beetle into something extraordinary. The dried up beetle carcass above is one of the prettiest examples of iridescence I’ve ever seen up close. The beetle is a golden buprestid (Buprestis aurulenta) which lives in my biome. As larvae they spend two to four years mining through recently dead conifer trees, whether that tree is rotting on a forest floor or part of your new coffee table, earning themselves the title of ‘pest’.

Once they morph beyond the furniture-eating stage, their exoskeleton matures to an iridescent green with brass coloured fringes around the wings. Strung together, these beetles would make a necklace suitable for a fancy ball, and I’m not the first to consider an iridescent beetle fit for a resplendent occasion. Ellen Terry, perhaps the most famous actress in the Victorian era, wore a green dress decorated with iridescent beetle wings to play Lady MacBeth in 1888. The dress must have looked stunning under the stage lights.

Beetles don’t hold a monopoly on iridescence; in fact, a diverse group of animals have independently evolved with their own version of colours with variable intensity and hue depending on the angle they are viewed (1). Examples abound in my own yard from dragonflies, butterflies to hummingbirds and more.

Why be iridescent?

An obvious use of iridescence is to communicate. A flash of bright colour might scare a predator away or say “I’m poisonous, so don’t eat me”. Or an animal could produce a flashy show to attract a mate (these guys put on the best show, but unfortunately don’t live in my yard). An untested hypothesis is that iridescence may help a school of fish or a flock of birds organize themselves (1) – another form of communication. For example, the iridescent patch on a mallard duck‘s wing may be a cue to help them fly in the same direction.

Counterintuitively, iridescent colours can also be used to hide (1), which explains why little fish like herring and sardines are so shiny – when looked at from below, their shininess blends with the shininess of the ocean surface. Or an animal can use iridescence to pretend to be something else – what looks like a drop of dew on a leaf might actually be a green leaf beetle (golden buprestids are probably to big to do this).


A drop of water on a leaf – or something else?

Interestingly, some instances of iridescence evolved before the organisms bearing iridescent structures developed the ability to see (1). One theory as to why iridescence evolved is that the structures that can create iridescence also create strength – so perhaps the iridescence of the golden buprestid is a side-effect of building a strong exoskeleton. We know these exoskeletons last, as fossilized beetles as old as 49 million years have been found that are still iridescent (2).

Now, I’ll need to find about a thousand more golden buprestids to make a ballgown of my own.


Golden buprestids from the collection of the Royal British Columbia Muesum



(1) Doucet, M. and M.G. Meadows. 2009. Iridescence: a functional perspective.

(2) Parker, A.R, and McKenzie, D.R. 2003. The cause of 50 million-year-old colour. Proc. R. Soc. B. 270, S151S153. 

A trio of unrelated topics


A different funky ladybug -a Multicoloured Asian Lady Beetle

I don’t have a complete blog post in my head, so here are three unrelated topics I’ve been thinking about.

1. Last week, during our first cold snap of the year, I found another unique lady bug active in the garden (first one here). This one was much blacker that others I’ve seen. It turns out its a Multicoloured Asian Lady Beetle (Harmonia axyridis), first introduced to North America in 1916 to control aphids and is now well established.

2. Here, the rainy season is well underway. I actually don’t mind the rain, but then, I grew up in this environment. I recently read an interesting article (check it out here) about the scent of rain. Like instruments in an orchestra, the smell of rain originates from microorganisms emitting organic compounds as perhaps a collective call for help, plus notes from plant oils and fungus. One of the culprits is the compound geosmin, whose name literally means ‘smell of the earth’. Geosmin was isolated and identified in 1965 – yet I don’t think it is in any of the commercial grooming products (like deodorant) that claims a rain scent as they never actually smell like rain.

3. The colours in nature never cease to amaze me, which is why I’m currently reading ‘Colour: Travels Through the Paintbox’ by Victoria Finlay for a second time (I’m on the chapter about red dyes from insects). Blue in nature is a particularly interesting colour as natural blue pigments are rare. When I first started this blog, I wrote about blues here, here, here, here and here (clearly this is a favorite topic of mine). I recently stumbled upon this article which is a great explanation of the topic.

some eggy tidbits


Both of these chicks turned out to be hens.

The longer days have resulted in a glut of eggs around here – hens I had thought were well into henopause have started laying again, and a chick I got last year that left me wondering if it was a rooster has proven she is a hen (I’d happily keep a rooster, except that in my urban area that wouldn’t be fair to my neighbours). With extra eggs in the fridge, I’ve been thinking a lot about eggs.

Ever dropped an egg? It turns out an egg can be repaired – good news for Humpty Dumpty! Even better news for a Kakapo egg as these ground dwelling parrots are critically endangered. Here‘s a story about an accidentally crushed Kakapo egg that was repaired, then hatched.

Or, considering Easter is approaching, how about natural egg dying? These natural, homemade dyes look great – especially the blues and reds. Beet juice creates a great red/purple/pink range of colours. Perhaps not really appropriate for dyeing eggs, crushed cochineal insects also produce a great and non-toxic red dye that is found in all sorts of processed food. As a slight tangent – up until roughly the 1950s, cochineal was the dye used for British army uniforms. This dye gets listed under a number of different names such as ‘natural red 4’ or ‘red #40.’

In general red colouring in food causes me some concern, a while back I took a look at the surprisingly long list of red dyes in a brand of iron pills that my doctor recommended I take. I have no biological need for cadmium, yet it could be found in those iron pills (among other unnecessary things). Apparently, cerium can be used as a non-toxic alternative to cadmium. However, I’ve since found iron pills with no colouring at all.

As a final note: check out these finches playing the guitar.