Part 3: Blue Feathers

Here is part 3 of my 4 part series on the scattered blues. Check out part 1 here and part 2 here.

Blue feathers have evolved in many species of birds. A blue jay's plumage is an excellent example with blue and white. You can see the black and blue of a Steller's jay in your own backyard. A male mountain bluebird has blue plumage of this type along with the head feathers of the male lazuli bunting; both can be found in central British Columbia. We know that feathers don't contain blue pigment, so the colour must be a result of the feather's structure.

In the late 1800s, just after the discovery of Rayleigh scattering, naturalists used this new concept to explain why blue feathers were blue. Since they didn't have the tools to examine the nanostructure (structure in the order of a billionth of a meter) of a feather, naturalists assumed that within the feather there existed transparent cells full of particles that were tiny enough to create Rayleigh scattering. Like the sky, blue light would be more efficiently scattered. These transparent cells would also contain pigments to absorb the longer wavelength colours. As a result, to our eyes these birds 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 what direction the naturalists looked at them, the assumption that their colour was formed through Rayleigh scattering seemed valid. But, 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 came on the market – the electron microscope. Now naturalists could directly examine the internal nanostructure of blue feathers. Based on this first look, they interpreted the internal feather structure to contain randomly spaced objects. This meant scattered light would be incoherent leading giving support to the hypothesis of Rayleigh scattering. It took decades of further research to change this hypothesis and in the mean time many textbooks were written explaining that blue feathers were the result of Rayleigh scattering. By the 70's, scientists 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.

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 damaged feather would look almost black. So if you are lucky enough to find a Steller's jay feather, take care of it.

Thanks to G. Hanke for the photo of mountain blue birds.