I never saw a purple cow,
I never hope to see one ...
(Gelett Burgess, 1866-1951)
Purple crows, now, those I have seen. So have you. But the light has to be just right.
|See the hints of purple and blue on the wings?|
Crows are black, true. But their wings are also iridescent, glowing, when the light hits at certain angles, in tones of violet, blue, and sometimes even green.
The plain black colouring is produced by melanin, the same pigment that gives our skin, hair, and eyes their colours, and protects us from sunburn. In the crow, it also reinforces the feathers:
Melanin provides more than just coloration. Feathers that contain melanin are stronger and more resistant to wear than feathers without melanin. Feathers without any pigmentation are the weakest of all. Many otherwise all white birds have black feathers on their wings or black wingtips. These flight feathers are the ones most subject to wear and tear. The melanin causing the tips to appear black also provides extra strength. (Cornell Lab of Ornithology)The extra flash and glitter of the crow's wings is the product of a structural arrangement, in which light passes through several microscopic, semi-transparent layers, which absorb and reflect it differently according to the position of the viewer. We can see how that works, looking at the effect of an oil sheen on water, producing swirls of colour in the sunlight. This same arrangement gives us the ruby throat of a hummingbird, the purple and green spots of a starling in summer, or the emerald green of a male mallard's head.
|He's black, with just a touch of purple, and red flecks on grey where the sunlight hits. (Look at the photo full size.)|