Comments on: Where feather colors come from.

Where feather colors come from.

metastability — Thu, 05 Feb 2009 07:52:51 -0800

"Unlike virtually every other feather color, no pigment turns feathers blue. We've known that for decades. Instead, it's long been thought that a layer of cells on blue birds' feathers reflected light at blue wavelengths, similar to the phenomenon that makes the sky blue." Now, however, scientists have another explanation.

An interesting article about where birds' colors come from. See also interference.

By: jessamyn

jessamyn — Thu, 05 Feb 2009 07:57:52 -0800

As a total bird nerd, I found this really interesting. I wonder if I feed the goldfinches near me some purple seeds I can make them change color.

By: DU

DU — Thu, 05 Feb 2009 08:06:34 -0800

"Now"? Interference has long been an explanation of, for instance, peacock feathers and other "shimmery" ones. My college physics text (my edition published no later than about 2000 but probably more like 1995) invoked this explanation for bird feathers in the chapter on light waves. It's the same thing that makes an oily puddle have a rainbow. And I don't even know what this is supposed to mean: a layer of cells on blue birds' feathers reflected light at blue wavelengths, similar to the phenomenon that makes the sky blue. What "layer of cells" makes the sky blue? And what's the difference between a blue pigment and a layer of some substance X such that it "reflects light at blue wavelengths"?

By: Mister Cheese

Mister Cheese — Thu, 05 Feb 2009 08:37:29 -0800

Just from reading that article, its not a substance that reflects blue light, but the structure of the feather: "In the cloudy zone of blue feathers, the melanin and air cavities are so close that the distance between them is shorter than a wavelength of light, according to research by Richard O. Prum of Yale University and his colleagues. When scattering elements are this small, they interact with light through a process called constructive interference." They go on to explain that the feather would look gray in the shade. I'm pretty sure a pigment would retain it's color. I'm no light or color expert; maybe it's really not so unique as they describe. But I'm inclined to think so. Can't speak to the recentness of the discovery.

By: DU

DU — Thu, 05 Feb 2009 08:45:27 -0800

Just from reading that article, its not a substance that reflects blue light, but the structure of the feather: Right, I get that. The post mentions 3 theories: 1) Pigment 2) "layer of cells that reflect light at blue wavelengths" 3) This "new" theory about interference I'm asking what the difference is between #1 and #2 and how #2 is at all related to the sky.

By: vacapinta

vacapinta — Thu, 05 Feb 2009 08:51:01 -0800

2) is Rayleigh scattering which is a different mechanism than pigmentation. This article here, much more clearly written, may explain it better to you, DU. I don't know about your prescient college physics text, but the physical mechanism behind coloration in bird feathers has only been recently understood (see this 2003 paper)

By: Mister Cheese

Mister Cheese — Thu, 05 Feb 2009 08:56:05 -0800

Ah, sorry, I misread you. Yeah, I have no idea what they're talking about. I guess you could read it as that there's something about the cells that is not pigment based that reflects blue light. But who knows what that is. I agree that the sky analogy is really out there. Not a good way to explain things.

By: Mister Cheese

Mister Cheese — Thu, 05 Feb 2009 09:07:14 -0800

Hey vacapinta, am I reading that the difference with bird's feathers is that it's a coherent scattering of light rather than incoherent? So the old theory was that a layer of cells scattered light like the sky: "the reflecting surfaces" were "randomized positions relative to the incident light." But they've recently discovered details that lead them to believe that the reflecting surface is in fact highly structured, leading to a coherent scattering. I'm going to start reading more about this stuff!

By: DU

DU — Thu, 05 Feb 2009 09:16:13 -0800

I don't have my 5th edition Halliday & Resnick (1996) here but I do have the 7th (2005). It mentions butterflies and "color shifting ink" but not peacocks. I also remember the former from the 5th but not the latter. Possibly peacocks were never in there, possibly it was replaced with ink, possibly I just extrapolated it myself (seems pretty obvious if you know about butterflies) or possibly I learned about peacocks later and conflated it with butterflies.

By: painquale

painquale — Thu, 05 Feb 2009 09:23:00 -0800

Really great article. Thanks for posting.

By: winna

winna — Thu, 05 Feb 2009 10:38:01 -0800

I went back to Frank Gill's Ornithology, the textbook I had in my ornithology class in 1998, and it says that particles of melanin in the surface cells on the feather barb are the significant factor in creating blue. This, according to the old theory, caused longer wavelengths of light (red and yellow) to pass through the surface layer to an absorbent melanin layer below, leaving the blue wavelengths to be reflected. The theory then was that the size of the melanin particles determined whether the feather was green or blue, with larger melanin particles shifting the wavelength toward greener blues. And now it sounds like they think the 'absorbent melanin layer' itself is responsible for the blue coloring. Neat.

By: spicynuts

spicynuts — Thu, 05 Feb 2009 20:06:41 -0800

I don't have my 5th edition Halliday & Resnick (1996) I could probably find this for you in the conference room down the hall from me at Wiley if you want.

By: Xoebe

Xoebe — Thu, 05 Feb 2009 20:42:17 -0800

Didn't some company come out with makeup that was based on this principle a few years ago? Part of the marketing was that the makeup would look different on each individual. From what I recall of it, it looked weird. I think it was eyeshadow and lipstick. I seem to recall a National Geographic article about this in the last couple of years as well. [Googles...] Yup! But more than a couple of years ago.

By: iamkimiam

iamkimiam — Thu, 05 Feb 2009 23:01:13 -0800

This is one of those totally great and absolutely underrated posts.