The graded blue colour on a Jay's feather

The blue wing-feather of a Jay will repay close examination with a hand-lens or a microscope. These feathers have a series of blue bands running across them at an angle. The only coloured pigment present is black melanin. The colour is usually only seen by reflected, not transmitted light. This is a good indicator of a physical colouring process.

a scanned jay's feather

The body of each barb consists of a milky-white matrix of keratin with tiny air cavities which, being smaller than the wavelength of light, scatter preferentially the blue end of the spectrum. The lovely gradation from deep blue to white is the result of melanin plates - which act as a black background - at varying depth within the keratin in a series of shallow steps along the barb. Where the black plates are absent, there are so many scatterings that all colours emerge equally like the white sky at the horizon on a clear day.

drawing of a jay's feather

As the melanin approaches the surface of the barb, there are fewer scatterings before the light leaves the surface and so the preference for blue becomes apparent. Right up against the black band, there are only single scatterings and the emergent colour is a deep blue - like the zenith sky from a high-flying aircraft (Fosbury 1984). (notes: 24 Feb 1984).

detail of barb

There are copious examples of this process at work in animals. The blue of some eyes, the mandrill's face, the necks of turkeys and guinea fowl and many different feathers are the result of Tyndall scattering against a dark background. Some of the animal greens, eg. lizards and frogs, are produced by scattering in the presence of a yellow pigment (Vevers 1982, Fox 1976, 1979).

Jay notebook page
Photograph (July 1999)
Scan (July 1999)