Living the Scientific Life (Scientist, Interrupted)

tags: Blue House Sparrow, Passer domesticus, birds, blue feather color, plumage color, refraction

I contemplated giving this bird to you as the daily Mystery Bird, but decided that you'd all probably riot, so instead, I am going to identify this bird so we can discuss it. It's a House Sparrow, Passer domesticus. (I have received dozens of emails telling me I'm a dumbshit: this is clearly a Eurasian Tree Sparrow, Passer montanus, thus, my wavering ID).

House Sparrows were introduced in various places around the world where they have become established. Because House Sparrows are invasive species, are poor singers and are small, plain and dirty-brown in color, they are generally widely hated.

So let's pretend that you were sitting at your breakfast table, sipping coffee while looking at your bird feeder, watching a flock of house sparrows pigging out, as usual. And let's pretend that you suddenly noticed that one member of the flock was blue! What would you do?

Fortunately for us, this happened to Richard Shears, who grabbed his camera and photographed the bird above. That bird recently appeared at Shears' bird feeder in Sydney, Australia with a flock of its normally colored companions. Shears managed to snap several photographs and, as you can see from the above image, this bird, which is probably a young male, is not dyed or otherwise painted in any way, nor is the image photoshopped.

So far, the experts are amazed by this bird's color, but no one has, to the best of my knowledge, offered an explanation for it.

How did this bird develop blue plumage? Since blue pigments have so far proven elusive for birds to manufacture themselves, we know it isn't the result of the bird suddenly being able to create it naturally. But blue birds exist, so how do they become blue? I wrote a piece about schemochromes -- structural plumage colors -- that will provide you with an in-depth look at blue feather color. In short, feathers create blue color by refracting light -- it's a physics trick that results in the feathers creating the appearance of being blue because they reflect blue light into the viewer's eyes.

My hypothesis: this bird developed a mutation in one or more of the genes that govern the structural development of its feathers. This change caused the feather structure to be altered so it reflects blue light, thereby making the bird appear blue -- which is true for all blue birds. I am not sure if anyone knows precisely how many genes need to be altered to produce blue feather color, but based on the sudden appearance of this blue bird, I'd guess that it's only one or perhaps just a few, at most.

But now that blue color exists in this species, this raises all sorts of interesting possibilities. Will females suddenly prefer to mate with blue-colored males over normally-colored males, now that they have the choice of blue plumage color? How will this color mutation affect the social, behavioral and evolutionary dynamics of this sparrow population in Australia?

What is your hypothesis about the origin of this bird's blue color? What do you think might happen with regards to the evolution of the species in Australia?