to post a photo of something alive that
meets a certain criterion, giving us an
opportunity to think outside our normal
posting topics and, often, learn something new! Metallic, this week's prompt, is easier to find in beetles than in plants! I've always admired that color I call "bug green," that was popular in cars a few years back.
Today, we'll venture into birds, where metallic plumage is an accessible option. It turns out there are three basic techniques birds use to create the display colors we appreciate especially during breeding season each year: pigment-based colors, structural colors, and cosmetic colors. Among the pigments, carotenoids produce yellow, orange, and red (as they often do in butterflies, flowers, and, well, carrots); melanins produce browns, blacks, and grey, not too spectacular sometimes, but forming the background against which the showier colors are displayed.
Melanins are also critically involved in the production of structural colors, serving as layers in thin-film reflectors or to absorb incoherently backscattered light from reflective keratin and air matrices (Prum 1999, as cited in Shawkey & Hill, 2005). Nano-scale reflective tissues, they add, usually produce UV-blue, white or iridescent coloration.
I thought sure the Black-billed Magpie above would be happy to demonstrate, but he only looks blue. He/she posed in the sun this a.m., giving a little better show.
Because his/her normal magpie iridescence was not adequately displayed in these photos, I turned to a feather source nearer at hand, if less exciting: our mixed-breed flock of domestic poultry. Beaks, here, being a Black Australorp rooster, was willing, and iridescence, or metallic hues, does seem to be best displayed in black feathers. Even this close-up can't do justice to the structural colors created by the intricate design of feathers. George refused to come out from under the juniper to pose, but I wish you could see his iridescence, not confined to blue-green, but venturing into mahogany and rust.
Here's a Partridge Rock hen, capturing a little of the mahogany color George displays so well, along with the traditional iridescence, all against the melanin background feather pattern, somewhat more subdued.
For comparison, a Buff Orpington hen demonstrates complete lack of metallic iridescence. According to Shawkey & Hill, some carotenoid displays (notably in the American goldfinch), though pigment based, depend upon white structural tissue to achieve the brilliance we expect in their vivid yellow.
And cosmetics? As we might expect, they are substances (from oil glands or soil, e.g., iron oxides) externally applied by birds to their feathers, to boost their appearance and attractiveness to potential mates. Parrots and pigeons, among others, use this approach.
All these techniques for creating display colors in birds have a metabolic cost, and must also have an adaptive advantage as payback, without which our world would be less metallic and far more drab.
Shawkey, Matthew D. and Geoffrey E. Hill. 2005. Carotenoids need
structural colours to shine. Biol. Lett. 1, 121–124; doi:10.1098/rsbl.2004.0289 Published online 16 May 2005.