Why Are Blue Birds and Blue Butterflies Blue?
Grind Up the Feather and the Blue Vanishes
Grind up a red feather and you get red powder. Grind up a blue feather and you get a dull grey or brown powder. Most blue birds and butterflies have no blue pigment at all. Their blue is not a "colour material". It comes from tiny shapes, too small to see, built inside the feather.
A kingfisher darts off a branch by the river. Its back flashes a jewel-like blue. The morpho butterflies of South America, which you may have seen in a field guide, also shine with a metallic blue.
But hold a blue jay feather you found in the park up to a window, and the blue is gone. It looks plain brown.
If something is coloured, it should look the same colour from every side. So why does a blue feather lose its colour depending on how you look at it?
Two kinds of "shape" make things look blue
A butterfly's scales hold many thin plates, each about as thick as a wavelength of light, stacked with gaps between them. Only the colour whose wave crests line up after bouncing off each plate comes back strongly.
Inside the blue feathers of birds such as kingfishers, air bubbles a few hundred times thinner than a hair are thought to be packed in at nearly even spacing. Only blue light that matches this spacing reinforces itself and comes out.
Neither one paints on a colour. Both use the wave nature of light, through shape. So if you break the shape, the blue disappears.
Butterfly blue: thin plates line up the crests
Sunlight looks white, but it mixes lights of different wavelengths, from red to violet. A soap bubble looks rainbow-coloured because light bouncing off the front and back of the film overlaps, and certain colours reinforce each other.
A morpho butterfly's scales are thought to be built like many such films stacked together. Look at Figure 1. When the spacing between plates is just right, blue light reflected from every plate leaves with its crests lined up. Red light is weakened, because its crests and troughs cancel.
Slide the control in Figure 1 to the right and the colour shifts from blue to green. In fact, if you drip alcohol onto a morpho butterfly's wing, the gaps fill with alcohol and the blue turns greenish. When it dries, the blue comes back. This is the clearest proof that the colour is set by shape.
Colours made by stacked plates change with the viewing angle. Seen from the side, light travels a different distance through the plates. The neck of a pigeon and the wing of a jewel beetle shine in different colours at different angles, and the cause is thought to be a related mechanism.
Bird blue: a sponge of bubbles strengthens only blue
Kingfishers, blue-and-white flycatchers, red-flanked bluetails: the feathers of Japan's blue birds are thought to be built a little differently from a morpho butterfly. Inside the feather's branches is a sponge-like layer full of tiny air bubbles, and the spacing of those bubbles is nearly even.
The bubbles do not form a regular lattice, so the colour changes little when you change the angle. Even so, blue light matching the bubble spacing reinforces itself, and other colours cancel out. Under the sponge lies a layer of black pigment (melanin). It is thought to soak up stray light and make the blue stand out.
Look at Figure 2. The red feather on the left gets its colour from pigment, so it stays a red powder when ground. The blue feather on the right loses its sponge shape when ground, and only the dark pigment underneath is left. When you hold it up to the light, it looks brown because you are seeing light that passed through, not the blue that bounces back.
How do other living things make blue?
Birds and butterflies are not the only ones that can hardly make blue pigment. In the animal world, blue pigment is thought to be limited to a few butterflies, fish and the like. The blue of frogs and lizards is also mostly a colour from the arrangement of tiny crystals.
Plants, on the other hand, make blue with pigment. The blue of morning glories and hydrangeas comes from pigments that can also be red or purple. They are thought to lean toward blue depending on the acidity of the sap and how they bind to metals. Even plants struggle to produce a pure blue.
The iris of a blue-eyed person is thought to have only a small amount of brown pigment (melanin), and no blue pigment at all. Fine fibres inside the iris scatter a lot of blue light, as the sky does, so the eye looks blue. The mechanism differs a little from a bird's, but the point is the same: blue comes from shape and how light is scattered.
Many birds are thought to see ultraviolet light, which humans cannot. If the spacing of the bubbles or plates in a feather is a little narrower, the light that reinforces itself is ultraviolet. A feather that looks plain to us may show up as a bold pattern to other birds.
Summary
Most blue birds and butterflies have no blue pigment. Plates and bubbles inside the feathers and scales, about as fine as the wavelength of light, make only blue light reinforce itself and return. So when you break the shape, the blue disappears. Animals took a completely different road from plants, which make blue with pigment.
A blue feather is not painted with colour.
It lines up the "crests" of light waves, using shape.
For how films make colour, see Why Soap Bubbles Look Rainbow-Coloured. For how fish use the same stacked plates to make mirrors, see Why Sardines and Cutlassfish Shine Silver. For how blue light scatters, see Why the Sky Is Blue. For plant pigments, see also the article on flower colours.
- Look at a pigeon's neck near the station or in the park, changing your angle as you watch. Green and purple seem to trade places. With pigment, the colour would not change with angle.
- If you find a blue feather, such as a jay's or an azure-winged magpie's, hold it up to the light. If the blue vanishes and it looks brownish, that is a colour of shape.
- Stretch soap solution across a wire loop and watch the colour change as the film gets thinner. You can see how thickness alone sets the colour.
Do not touch fallen feathers with bare hands. Use gloves or a plastic bag, and wash your hands afterward. Do not catch wild birds or go near their nests.
For those who want more: terms, formulas and links to textbooksEach part is labelled with its level, from middle school science to university courses
- Middle schoolCovered in middle school science
- High schoolCovered in high school physics
- High school+Advanced high school material, or textbook sidebars
- UniversityUniversity courses (optics, biophysics) not taught in high school
- ResearchTopics researchers are still investigating, not yet settled even at university level
Middle schoolTerms: this phenomenon has a name
- Structural colour: colour produced not by pigment but by structures as fine as the wavelength of light, which make light reinforce itself or scatter.
- Pigment colour: colour produced by a substance (pigment) that absorbs light of certain colours. It survives grinding.
- Interference of light: when two or more light waves overlap, crest meets crest and they strengthen each other; crest meets trough and they weaken each other.
Middle schoolHigh schoolCheck with a formula: what colour does a butterfly scale send back?
In a structure of alternating plates and gaps, the colour that reinforces itself is the one for which the path difference between light reflected from neighbouring plates equals one wavelength. Here we calculate the wavelength of the returned colour for light shining straight on.
| In symbols | λ ＝ 2 ×（ n1 × d1 ＋ n2 × d2 ） |
| In words | Reinforced wavelength = 2 × (refractive index of plate × plate thickness + refractive index of gap × gap thickness) |
| Where it comes from | It comes from the interference condition (the Bragg condition for multilayer films). When the "optical path" of light going there and back across one plate-and-gap pair is a whole number of wavelengths, light reflected from every plate has its crests lined up. |
| Symbol | Meaning and unit |
|---|---|
| λ | Wavelength of the light that reinforces itself and returns (nanometres) |
| n1 | Refractive index of the plate (chitin) (no unit) |
| d1 | Thickness of one plate (nanometres) |
| n2 | Refractive index of what fills the gap, such as air (no unit) |
| d2 | Thickness of the gap (nanometres) |
| Refractive index of chitin | Said to be about 1.56 |
| Plate thickness (assumed value, similar to sizes reported for morpho butterflies) | 75 nanometres |
| Refractive index of air | 1.0 |
| Gap thickness (assumed value) | 120 nanometres |
| Refractive index of ethanol | Said to be about 1.36 |
| "Thickness as light sees it" of a plate | 1.56 × 75 ＝ 117 |
| "Thickness as light sees it" of an air gap | 1.0 × 120 ＝ 120 |
| One plate-and-gap pair | 117 ＋ 120 ＝ 237 |
| Reinforced wavelength (doubled for the round trip) | 2 × 237 ＝ 474 |
| When the gap is filled with ethanol | 1.36 × 120 ＝ 163.2 |
| One plate-and-gap pair (ethanol) | 117 ＋ 163.2 ＝ 280.2 |
| Reinforced wavelength (ethanol) | 2 × 280.2 ＝ 560.4 |
With air the wavelength is about 474 nanometres, which is blue. Filled with ethanol it is about 560 nanometres, a yellow-green. The formula confirms that just changing what fills the gap moves the colour from blue toward green. The slider in Figure 1 also calculates its colour with this formula.
High schoolHigh school+Thin-film interference, and why the colour changes with angle
High schoolHigh school physics teaches the interference of light reflected from the front and back of a thin film as "thin-film interference". The condition also takes into account that a wave's crest and trough swap (the phase shifts by half a wavelength) when it reflects from a surface of higher refractive index.
High school+When light enters at a slant, the path difference inside the film shrinks in proportion to the cosine of the angle of travel inside the film. So the more slanted your view, the more the reinforced wavelength shifts toward the short side (blue to violet). This is why morpho butterflies and pigeon necks change colour with angle.
UniversityThe "disordered arrangement" of bird feathers, and a steady colour
The sponge of bubbles in a bird's blue feather is not a regular lattice like a crystal. Even so, because the distribution of spacings between bubbles is narrow, it is explained that "coherent scattering" occurs, in which only scattering at a particular wavelength reinforces itself. Break the bubble arrangement into wave components (Fourier analysis) and a sharp peak appears at a certain spacing, which lets you predict the wavelength of the colour. This view connects to research on "quasi-ordered structures" and "photonic crystals", where colour appears even in disordered structures.
📖 Derivations and further reading: Structural coloration (Wikipedia, in Japanese) ／ Thin-film interference (Wikipedia, in Japanese)
ResearchWhat is still not clear
- Why don't animals make blue pigment? One idea is that blue pigment molecules are large and complex and costly to build, but the question is not settled.
- How does the bubble sponge form? It is thought that as the feather grows, protein and water separate on their own and form the bubble pattern, but the fine mechanism that makes the spacing even is still being studied.
- Can we copy shape colour and use it? Fade-resistant "pigment-free paint" and display applications are being researched, but making it as cheaply and in as large amounts as living things do is thought to be hard.
In other words, this article is also "an explanation of what is known so far". The thickness values for plates and bubbles vary with the species and the way of measuring.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science, Year 1: "Properties of light" | White light is a mix of many colours |
| High school | Physics: "Interference of waves", "Thin-film interference" | The condition for crests to line up, checked with a formula |
| High school+ | Physics (advanced) | Why colour shifts with angle |
| University | Optics, biophysics | Coherent scattering, photonic crystals |
| Research | Biophotonics | Why animals don't make blue pigment, how the bubbles form |
| ― | Links to daily life | Pigeon necks, soap-bubble colours, developing fade-free paint |
- Vukusic P., Sambles J. R. "Photonic structures in biology", Nature 424 (2003)
- Prum R. O., Torres R. H., Williamson S., Dyck J. "Coherent light scattering by blue feather barbs", Nature 396 (1998)
- Shuichi Kinoshita, 『生物ナノフォトニクス ― 構造色入門』 (Biological Nanophotonics: An Introduction to Structural Colour; in Japanese), Asakura Shoten (朝倉書店) (2010)
- Wikipedia (Japanese), "構造色" (Structural coloration)
Note: This article is a science explanation for general readers. The numbers given are rough guides to help you understand the mechanism. Plate and bubble thicknesses include assumed values and differ between species.