Monday, August 07, 2006

Animation of Luneau bird

EDIT August 9 '06 -- watching real birds fly these last few days (pileateds, great blue herons, etc.) it's clear that I missed one detail in both these movies: the way the tips of the outer primaries flex smoothly upwards during the downstroke. That will be incorporated into future sketches.

My original intent of putting together the Pileated animation was to then paint it up like a Pileated and fit it to the Luneau video frame-by-frame and see if it really can be made to match. I hit the first bump immediately. The flight mechanics and wing postures of the Luneau bird don't match up with that Pileated animation. So, I put together a new animation of the Luneau bird:

232kB quicktime movie

As before, the animation is at half speed so it will match a deinterlaced 30 fps digital video. It shows one "typical" wingbeat repeated three times; download it and play it in a loop, click through frame by frame, etc. The Luneau video provides no actual side views, so the side views in the animation are based entirely on interpretation and extrapolation from the rear views. As with the Pileated, at all positions in the downstroke it is the ventral surface of the wing that is visible in the rear views. This is going to be true of most any bird that is flying exactly away from and on level with the observer. And of course the Luneau video does not actually resolve any of the particulars about the shape of the bird, so I have used a normal Pileated body as the model to fit the flight mechanics to.

The stumbling blocks I hit when trying to match this bird up with Pileated flight:

Most glaring is the shape of the wings early in the downstroke. In every wingbeat, the Luneau bird holds its wings with a strong downward bend at the wrist from nearly the beginning of the stroke. The Pileateds sometimes showed bowing of their wings, but primarily in the first flap or two after launching from a tree while still making the transition from perched to flight; and most notable late in the downstroke. In contrast, the Luneau bird shows bowing strongly and early in each wingbeat. Indeed, the Luneau's birds wings appear to straighten towards the bottom of the stroke, at the same point where a Pileated's wings are most likely to bow.

The apparent length of the white underwing at maximum upward extension (frame 1 in the movie) is considerably shorter than the apparent wing length in the other frames. This patern repeats on every wingbeat in the video. I interpret this to mean that the wing is not fully extended in its uppermost position at the start of the downstroke. This consistent with the wing bowing observed in the frames immediately following this. During a rapid upbeat with the wings close to the body (as shown by both the typical Pileateds and the Luneau bird), the upstroke consists of two stages. First, the humerus (innermost wing bone) is raised whle the rest of the wing remains folded close to the body, oriented downwards. Then in a quick rotation, the outer wing is raised and extended. In the case of the luneau bird it appears that this second stage is not carried out to full extension; rather the outer wing reaches outwards (and probably forwards) to begin the downstroke without having been raised to a full upright position.

A final point is difficult to judge because of the fuzziness of the video. However, it appears that the Luneau bird might show a more pronounced rotation of the outer wing on the downstroke (probably a direct result of the bowing of the wings) than does the "typical" Pileated. When the bird is positioned most nearly in direct line away from the camera, the amount of white underwing exposed on the downstroke increases dramatically away from the birds body, sometimes giving the impression of two white orbs suspended in space on either side of a wingless body. This could simply be a function of motion blur which would also increase distally on a moving wing; it is hard to say.

Overall, the differences between the flight of the Luneau bird and that of the typical Pileated are of a magnitude similar to differences routinely used in the field to distinguish seabirds on pelagic trips; perhaps comparable in degree to the differences between Cory's and Sooty Shearwaters, or Wilson's and Band-rumped Storm Petrels. They are perhaps similar to what one might expect to see in two large woodpeckers that are superficially similar but in fact are in two different genera. Alas, differences of this magnitude per se are not generally sufficient to document high-level rarities without other supporting evidence. But they sure are suggestive and make it hard to simply agree with a declaration that the Luneau bird is a perfectly ordinary Pileated showing nothing at all that is inconsistent with this identification.

More to come, someday...


At 7:38 PM, Anonymous Anonymous said...

I believe the downward bend could be explained by motion blur in at least some of the frames rather than being an actual bent wing.

In any case look at the photographic evidence of what an IBWO wing looks like on the down stroke:

I see no hint of a bent wing.

At 8:28 PM, Blogger Bill Pulliam said...

The bent wing feature is highly persistent in the Luneau video, appearing on multiple frames of every visible wingbeat. I don't see how motion blur can create this. It is a real feature of the bird's flight; I don't believe there is any reasonable way it can be explained away as an artifact.

Whether or not this is a character that is typical of IBWO flight can't be determined, and I have made no such claim. It is not characteristic of typical pileated flight, and I repeat that this makes it inaccurate to state that everything about the Luneau bird is completely consistent with a normal PIWO, as many have.

The totality of the IBWO comparison material we have consists of the two well-known frames (the one overhead and the one launching from the nest tree), and two blurry images of more distant birds that have their wings strongly folded. It is very difficult to extract any meaningful comparison between these out-of-context stills associated with unknown flight behavior and from very different viewing angles versus the Luneau video frames. This is at least as dubious an endeavor as the often-criticised comparison of the Luneau wingbeat rate to that in the audio recording of a bird engaged in unknown behavior. We need to be consistent; we should not cherry pick and accept a comparison here and reject a similarly shaky comparison there. In particular I think you refer to this image:

That shows a bird that has just launched from a nest hole and is headed sharply downwards. We don't know if it is just intending to sail down to a nearby tree trunk, or if it is preparing for a long-distance flight. At no point in the Luneau video is the bird seen in a comparable flight situation. The Luneau bird is 1.25 wingbeats post-launch when first clearly seen, and it is never headed downwards. And, even after all these caveats, in fact the right wing of the Tanner bird (extended towards the camera) does appear to be flexed downwards at the wrist to some extent.

At 3:52 PM, Anonymous Anonymous said...

I really wish there were more photos of IBWO. It is hard to judge what's going on in those few launch images. Is there a bend or a twist. To me they look faily straight but that involves a fair bit of loose interpretation at this point. Lacking anything else we have to go with what we have for photos whether the context is perfect of not. The only other thing we have are people's descriptions and drawings.

The wings appear to be significatly wider toward the wingtips than they are near the body in multiple frames also but I don't think that reflects the geometry of the wing, it's a motion artifact. I think there is quite a bit of wing twisting happening as the bird arcs through the top of the stroke and starts the downstroke. I think at least some of the visible arc is created as the twist reveals different amounts of white and black. Maybe there's some bend in there also.

If you took a really long exposure image at light and took a flashlight you could create a curving line even though the flashlight is a "point". My sense is that something like that is contributing to the wing bend in the images. I think a PIWO's translucent primaries could be one source of such a "light". Not sure if a twisting IBWO wing could also do that. Need to do a little more searching.

At 4:10 PM, Blogger Bill Pulliam said...

It is very hard to figure out exactly what is going on with the wings in parts of the Luneau video, and it is going to get even worse when I ver get around to tackling the second half-second of it.

I think it is clear that no identification can be extracted from this video with the available comparison material that wil be generally accepted. What I am looking for are features that can be identified that contribute some possible measure of differentiation. I too wish we had much more IBWO comparison material. If the happy day ever comes that we get this material from contemporary sources, then we will be in a position to revisit all these Luneau features and examine if they are or are not IBWO features. Even if this wished-for new material comes from somewhere other than AR, it may finally shed definitive light on what really was happening in Arkansas.

We live in hope...


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