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