Several folks responded quickly to the previous post-query (if you haven't already seen it you may want to jump down to read that post first), and artist/birder Julie Zickefoose's explanation was as good and thorough as any of them:
"Here's what's happening in that photo, and in life:Thanks for the quick response Julie (and all others as well).
The famous (and only good) Arthur Allen photo of the flying ivory-bill was taken from beneath, with bright overhead lighting. As such, the white secondaries and inner primaries are illuminated and clearly read as white. The white lining of the underwing, which includes the underwing coverts and feathering along the ventral surface of the patagium, does not appear white in this photo because it is in shadow, and the light is not shining through it. If you look at any photo of a flying bird, taken from below and brightly lit from above, light is able to pass only through the flight feathers along the trailing edge of the wing, since there's only one layer of feathers there. Light really can't pass through a patagium, since it's heavily feathered, and there's skin and bone to further block that light. So, confusingly, this "wing lining" appears dark in the photo. But rest assured that Roger Peterson and other careful bird painters did get it right. And field guide plates emphasize local color rather than artifacts of light, because their mission is to show what color the bird actually is, rather than the color it may appear to be.
I invite everyone to look at my comparison plates of pileated and ivory-billed woodpeckers in the latest issue of Bird Watcher's Digest, just hitting the mailboxes and newsstands."
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