Monday, July 23, 2007

Once again...

Yes I know this passed tedious months ago.

To answer the questions asked by Sibley et al in the web extra in the current issue of Birding, plus point out a few somewhat misleading statement of theirs:

First, "they" collectively may not have emphasized the the "narrow -- and ambiguous -- black trailing edge," but Bevier as an individual (one of the als in the et al of the letter) certainly has, and Collinson made it a cornerstone of his published analysis. And indeed, in this self same letter they use a portion of the "black trailing edge" argument as the basis for their point number 4. It borders on misleading to suggest that this has not been an important part of the pro-Pileated analyses, even when one just limits oneself to peer-reviewed literature (which it is worthy of note that this "letter to the editor" itself is not).

OK, on the the questions:

1. In the second frame, why is the brightest white far from the body with little white close to the body?

Angle of presentation, perhaps? Outer wing oriented vertically and farther from the shadow of the body, inner wing oriented downwards and closer to the body. There are other frames where the inner wing is brighter than the outer wing, especially when it is oriented vertically and the outer wing is oriendted downwards.

2. How could a bird with white flight feathers largely disappear against a dark background (fourth frame)?

It doesn't. I see it there as clearly as in any other frame, which of course means not very clearly at all. A few pixels that look like a dark body, two small clumps of lighter pixels below this body that could be some white flight feathers, and a dark pixel or two below that, maybe. It looks to me like there are as many light pixels as dark pixels. Whatever, it's another "six pixel bird."

3. Why are the secondaries on the left wing black (fifth frame)?

Claiming that black smudge as the secondaries of the left wing is extremely tenuous. First, comparison of this and the next frame show quite clearly that the wing is NOT fully extended in this frame: note how much bigger the right wing appears in the next frame. Look at high-speed still photos of birds in flight that pull their wings in close to the body on the upstroke. At this stage of the stroke, the inner wing is extended upwards (hence the white bar shown on the right wing) but the outer wing is still folded and in the middle of a rapid rotation from downward to upward. So that black bar could be just about any part of either wing at this instant... if it is even real.

Which, it is highly questionable if it is. According to Bold Mistatement #1, "These patterns are repeated consistently in each wingbeat cycle through the video." This particular feature in fact occurs clearly only in this one frame on this wingbeat; at similar stages of other wingbeats these conspicuous "black secondaries" are not to be found. This suggests that these "black secondaries" are as likely to be an imaging artifact as anything else.

4. How could the partially white wingtip of an Ivory-bill produce the broad black band shown curving around the wingtip in the sixth frame?

It didn't. The image compression process produced this broad back band (see my extensive posts below for details). It is not a plumage feature of the bird.

5. How could the all-white secondaries of an Ivory-billed Woodpecker produce this image showing no white on the left wing (seventh frame)?

There is no left wing at all in this frame to my eyes. We might as well ask, how could the mostly black flight feathers of a Pileated have disappeared completely? In fact, even better, we should ask: How could the prominent "black secondaries" from the fifth frame have disappeared entirely by the seventh frame if they were real? Both black and white are resolved against the background (black body, white underwing) in other frames. Rather than inferring field marks from an invisibe wing, it seems better to conclude that a rapidly moving, out of focus wing viewed nearly edge-on disappears no matter what color it is.

Synopsis: for Pileated field marks we have gradations of shading on a strongly angled underwing, an invisible six pixel bird, a black smudge in one frame that does not appear consistently in other comparable frames, an image compression artifact, and an invisible, motion-blurred wing viewed edge-on.

And we conclude with Bold Mistatement #1 as mentioned above: "These patterns are repeated consistently in each wingbeat cycle through the video." Actually, items 1 and 4 appear more clearly in this wingbeat than in the others, and item #3 appears only in this one frame of this one wingbeat. They are not consistent; most are contradicted in other frames and other wingbeats within the video.


At 9:01 AM, Blogger Tucano said...

Hi Bill,

It seems to me that by publishing Sibley et al.'s letter and making their material available online at ABA's web site they have alo opened themselves to do the same for anybody wishing to post a rebuttal of Sibley's argument, here is your chance to have your stuff reach Birding readers.



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