[Tweeters] Another questions

Kevin Purcell kevinpurcell at pobox.com
Tue Mar 8 17:18:12 PST 2011

On Mar 8, 2011, at 2:17 PM, Christine Southwick wrote:

> What mechanism is used when birds/ducks bills change colors for breeding display? How does a Ruddy Duck's bill turn a bright blue, and then back? Or a starling's from black to yellow-orange? Also,does it happen all at once? One morning it isn't the next morning it is?

Ah, a curious question as I was watching American Robins recently and noticing the variation in bill and breast colors amongst them.

The change in the starlings bill results from the deposition of carotenoids in the bill (controlled by the presence of hormones related to breeding) that changes the color of the bill from black to yellow. It is hypothesized to be an honest signal of condition of the bird. It happens gradually as the breeding hormones turn on. How much it changes color depends (probably) on how much of the required carotenoids it eats as the bill changes color.


for Spotless Starlings (in Spain)


and American Goldfinch


Though some think this simple story is not complete


One other point as regards timing. The change in Starling bill color is gradual as the carotenoids accumulate. For carotenoid feather coloration a lot of people seem to assume that the birds get more colored as time goes on. This isn't the case. The carotenoid color (e.g. the red and yellow head and breast color in House Finches) is deposited on the feathers as they grow back after molt. So the coloration is set during a very brief time as the feathers grow and they reflect the state of nutrition of the bird at that point (for house finches this is also a time when fruits are available which may provide those carotenoids).

As for the RuddyDducks when I started this email I had a good (only slightly incorrect) guess but I now know why.

Clearly for the red or orange or yellow billed ducks, such as Mallard, I can see the carotenoid argument working. Black bills have eumelanin. Brown bills have phaeomelanin. But blue bills? Blue colors in birds are structural colors not pigmented colors.

The Ruddy Duck bill color change with breeding status from a grayish-blue to bright blue. In fact this seems to be roughly true for all blue or gray billed ducks their tone and saturation changes (i.e. they get "brighter") as they move into breeding state.

Others have noticed this before

e.g. http://elibrary.unm.edu/sora/Auk/v086n04/p0765-p0766.pdf



> Bill And Gape


> Breeding male distinguished by a bright-azure (sky blue) bill with a blackish nail (Hays and Habermann 1969). During nonbreeding season (mid-Aug to late Mar, early Apr), male bill color is gray-black. Female bill slaty.

Initially I suspected blue bill have the same non-iridescent blue mechanism that demonstrated by Geoff Hill in Steller's Jays. Eumelanin is deposited in a transparent keratin small granules about the size of wavelength of light. These strongly scatter blue light more than longer wavelength light (i.e. Rayleigh scattering). Behind that layer is a solid layer of eumelanin that absorbs the (green, yellow, red) light that wasn't scattered. So the bill (or feather or foot) appears blue.

The problem with this for color change in the keratin is you can't change the granule composition without growing a new bill. And ducks (other than Daffy Duck) don't molt their bills.

But in the case of the Ruddy Duck (and, I think, others with blue bills) they have a ramphotheca, a dermal covering over the bill. So changes in the dermal structure of the ramphotheca could change the "bill" color that's easier than changing the color of the underlying keratin.

A bit more searching an I found this


> Bird bills (ramphotheca) can also exhibit non-iridescent structural color. Bill color is stable all year in most birds, but during the breeding season, the bill of the male ruddy duck (Oxyura jamaicensis) changes from black to blue. The color is produced from coherent light scatter from parallel, quasi-ordered arrays of dermal collagen fibers (Prum and Torres, 2003a).

Which cites: Prum, R.O., and Torres, R.H. (2003a). Structural colouration of avian skin: convergent evolution of coherently scattering dermal collagen arrays. J. Exp. Biol. 206, 2409–2429.

Prum is a big name in bird coloration. And fortunately he puts his papers online where scholar.google.com can find them.


It's an interesting paper and has some very nice color photos.

He shows that you can get non-iridecent color from coherent scattering if the scatterers are "quasi-ordered" i.e. not totally regular in spacing with a similar structure to the one described above.

And he shows there are quasi-ordered scatters in the "dermal collagen" of the ramphotheca of the Ruddy Duck.

I presume the skin structure changes with the presence of breeding hormones but I'm not sure if that work to elucidate the mechanism of that has been done.

I strongly recommend reading Geoff Hill's "Bird Coloration" if you have any interest in bird coloration. Excellent book. For more details on Geoff Hill's work on carotenoids in House Finches see his book "A Red Bird in a Brown Bag". It's hard to find (but it's a good read).

Or read Prum's review paper on the mechanisms of bird coloration

Kevin Purcell (Capitol Hill, Seattle, WA)
kevinpurcell at pobox.com

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