Wing twisting explained

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In discussing the identification of the woodpecker in the Luneau video, my colleagues and I suggested that the extensive white visible in the wings could be accounted for, in part, by the natural twisting of the wings during flight. A bird flying away would show the underside of the wings during the entire downstroke. In the formal response and continuing in the recent Recovery Plan for the Ivory-billed Woodpecker (p. 44), those who believe that the video shows an Ivory-billed Woodpecker insist that these are “…novel interpretations of typical bird flight.”

I hesitate to post this, because I really don’t want to reopen the whole debate over the video, but this is not a novel interpretation of bird flight, and it has applications far beyond the debate over one blurry video. I think it is important enough, and basic enough, to deserve more discussion. Any artist trying to render a flying bird of any species has to understand the flexing of the wings in order to produce a believable image. Similarly, any birder trying to interpret the appearance of any bird they see in the field will benefit from understanding the natural curvatures of birds’ wings.

There are really two points to consider. The first is that, when a bird is flapping, the leading edge of the wing “leads” on the downstroke, so that the entire wing tilts forward. The two photographs below demonstrate this clearly.

Golden-fronted Woodpecker taking off with both wings angled strongly down as it flaps. Photo copyright by Chris Hirsch. Used by permission. Clicking the image leads to his website.
Pileated Woodpecker flying away with wings at the bottom of the downstroke. Both wings are "twisted" or cupped so that the underside of both would be visible from behind. Photo copyright Lillian Stokes, used by permission. Clicking the image leads to her blog post with the full set of photos.

The second point is that birds’ wings are not flat, but curved to form an airfoil shape. Fitzpatrick et al. (2006) state simply that “Pileated wings have more black than white on the ventral surface” which is true in a strict two-dimensional sense. But in the real world, when viewed from behind, the natural curvature of any bird’s wing foreshortens the secondaries and emphasizes the leading edge of the wing, as seen in the photo below.

Pileated Wodpecker flying away, showing how the curvature of the flight feathers leads to foreshortening that makes the underwing appear mostly white, even though it's actually mostly black. Photo copyright Lillian Stokes, used by permission. Clicking the photo links to her blog post with more photos.

Understanding these fundamental aspects of wing mechanics is key to understanding the identification of the bird in the Luneau video, as well as interpreting the varying appearance of any bird in flight.

Links to the Luneau video and the debate over its identification can be found here.

The photographs of Pileated Woodpeckers above are from a fantastic series by Lillian Stokes. You can see the whole set and read the story behind them at her blog.

6 thoughts on “Wing twisting explained”

  1. Indeed, in downstroke, the leading edge of the wing leads the trailing edge, and the part of the wing closest to the bird’s body (closest to the large breast muscle) leads the tip of the wing (well illustrated in the photo of the Golden-fronted woodpecker). And, indeed, the entire wing is curved during downstroke (the degree and contours of the curvature changing throughout the wingbeat).

    Thus, I suspect everyone agrees that when viewing a bird from behind – and slightly below – the bird, the underside of the wings will be most visible. However, we rarely view a bird perfectly from behind, or only from behind. The bird’s flight course varies, and thus the bird is often quartering away from the viewer (or even “half-quartering” away). If the bird is gaining altitude in a steep ascent, even if viewing perfectly from behind and with maximum wing twisting, we will still mostly see the topside of the wing.

    That’s the case with the bird in the Luneau video – it is often quartering away from the camera, so that the most visible part of one wing is the underside, but the greatest part visible of the other wing is the topside – very nicely illustrated in the Lilian Stokes photograph.

    Note also the pattern of white, gray and dark on the underside of the left wing in Stokes’ photograph. The extensive white underside is rendered somewhat dark where the top side plumage is dark.

    Finally, because it relates somewhat, but I haven’t seen it mentioned elsewhere, one of the video artifacts in the Luneau video is a “dark halo” adjacent to light colored objects. This is especially noticable around the yellow canoe paddle handle. I believe a similar dark halo video artifact surrounds some of the birds white plumage.

    1. Erik, Thanks, I just wanted to establish the fact that curving and flexing of wing feathers can have a profound effect on what we see on a flying bird. I’m glad to know that you agree.

    2. Erik, I have addressed the edge artifact phenomenon in the Luneau video extensively on my blog. An overview of my analyses is at http://bbill.blogspot.com/2009/02/summary-of-my-luneau-postings.html

      Video analysis is a relatively new thing in the birding world. We are still figuring out how to best approach it. The ability to look at dynamics of motion and see a bird from multiple angles and in multiple poses is video’s obvious greatest strength; the poor quality of the individual images it equally obviously its greatest weakness.

  2. Timothy Barksdale

    Dear David,

    This is both an invitation and a challenge. In other emails I have written to you, perhaps to an older email address, you have not responded. Be that as it may, I openly invite you to a debate about the identification and the “reality” of what was going on with the Ivory-billed Woodpecker reports from 2004 to 2005 in Bayou deView, outside of Brinkley, Arkansas.

    I would propose that we do this in a setting which is conducive to fund-raising and for the purpose of the film, which I am finishing in 2011 – the Last Dance of the Prairie Chicken. This at least is a species which can -with out a doubt – be saved by our actions for prairie preservation, grassland restoration, and a shift in economic trends which seems to be able to
    help these great birds.

    I invite you to suggest a series of dates and locations to conduct this debate. We should agree about the rules ahead of time just as any good politicians would do. Hopefully, as fellow birders, this will be much easier to accomplish.

    VEry Sincerely,

    Timothy Barksdale
    Managing Member, Birdman Productions LLC

    1. Hi Tim,
      Thanks for the invitation/challenge. I think the debate over the Arkansas evidence is becoming less relevant with every month that passes, but I’d be happy to debate the evidence here on my website, and I’d be happy to help promote Prairie-Chicken conservation. I don’t think it’s a good idea to link the two.

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