- Video analysis of the escape flight of Pileated Woodpecker Dryocopus pileatus: does the Ivory-billed Woodpecker Campephilus principalis persist in continental North America?
On January 28 and February 5, 2006, David Nolin (DN) video-recorded Pileated Woodpeckers Dryocopus pileatus at a bird-feeder in Dayton, Ohio, USA. A Hi-8 Sony Handycam was used, hand-held, at approximately 5 m from the feeder. Birds on the tree trunk were alarmed by movement, and their escape flights recorded. Four escape flights were captured that approximate to that recorded for the putative Ivory-billed Woodpecker Campephilus principalis by Luneau in April 2004 and published in Fitzpatrick et al . The videos are not directly equivalent because the Pileated Woodpeckers made only short escape flights to nearby trees, whereas the putative Ivory-billed Woodpecker in the Luneau video showed little sign of coming to rest before being lost from view. Nevertheless, interesting comparisons can be made.
Wingbeat frequency of Pileated Woodpecker
The woodpecker in the Luneau video maintains a steady rapid wingbeat rate of 8.6 beats s-1 for at least 8 wingbeats , a figure that was confirmed by independent analysis during preparation of this paper. The Pileated woodpeckers in DN's video do not do this – after initial rapid flapping immediately after take-off, they settle into a more relaxed level flight. As shown in Tables 1 and 2, although the mean wingbeat frequencies of the Pileated Woodpeckers in DN's video are slower than the 8.6 s-1 recorded for the bird in the Luneau video [1,3,5] the first four wingbeats, the initial escape response, are faster than those claimed for Pileated Woodpeckers in the literature [1,3,5]. For the four escape flights, the mean frequency values for the first four wingbeats are 7.1, 6.7, 8.6, and 8.0 s-1, respectively. The 8.6 beats s-1 of the bird identified in the Luneau video, while consistent with the limited data (n = 1; see Discussion) for Ivory-billed Woodpecker, is equally consistent with Pileated Woodpecker in its initial escape flight. The bird in the Luneau video maintains a frequency of 8.6 s-1 for the next four wingbeats too, whereas the Pileated Woodpeckers recorded here all slowed their flight as they prepared to land in nearby trees. There are no data to suggest whether Pileated Woodpeckers can maintain a wingbeat frequency approaching 8.6 s-1 for eight or more wingbeats, like the bird in the Luneau video. It remains possible that the flight pattern of the bird in the Luneau video is unusual for Pileated Woodpecker, but a frequency of 8.6 s-1 is consistent with a Pileated Woodpecker gaining initial speed and height in escape flight, and by itself cannot be taken as strong evidence that the Luneau video bird was an Ivory-billed Woodpecker. This is discussed further below.
The video of Pileated Woodpeckers in flight was obtained in avi format, decompiled and examined frame by frame. Comparisons of Pileated Woodpecker images with key images of Luneau video are shown in Figures 1 and 2, and suggest a genuine resemblance between the bird in the Luneau video and a Pileated Woodpecker. Analysis is complicated by the different digital processing of the two videos, and in the case of the Nolin videos it is important to concentrate only on those frames or part-frames where apparent plumage features are not an artifact of blurred images. Thirty-six frames from the fourth example of Pileated escape flight, which most resembled the flight path of the Luneau video bird, were analysed systematically frame by frame. They represent seven complete wingbeats (1.20 s from the middle of the second wingbeat to middle of wingbeat 9) and were directly compared frame-by-frame with the equivalent fields (middle wingbeat 2 – middle wingbeat 9) of the Luneau video. This comparison is shown in Figure 3. The images of the birds are not identical, but in every frame of the 36 frames available, there are sufficient similarities to suggest that the bird in the Luneau video is consistent with the known Pileated Woodpecker. Further comparisons of the Luneau bird with the other three Pileated escape flights recorded are presented in the supplementary material (see Additional file 1).
1. Pileated Woodpeckers flying near-horizontally away from the observer show much more white in poor-quality video than would be expected from their general plumage pattern. They present an appearance of a black-bodied bird with largely white wings and black wingtips, very similar to the bird in the Luneau video; compare in particular Figure 1B, frame 758, with Figure 1A, frame 283.3. The expected appearance of the upperwing of Pileated Woodpecker – mostly black with a small white patch at the base of the primaries – is often not seen, and is only clearly resolvable when birds are flying near-vertically before landing on a tree trunk; something the bird in the Luneau video did not do.
2. The black trailing edge to the underwing of Pileated Woodpecker is often very inconspicuous and may disappear completely. Due to motion and flexion of the wing, the black trailing edge is much more obvious towards the wingtips. This produces an apparent plumage pattern that matches the patterns shown by the Luneau video bird (compare Figure 1B, frames 175 and 457 with Figure 1A, frames 300 and 416.7). In many frames of Pileated Woodpecker, a black trailing edge to the wing is discernable (though due to bleeding of white as a video artifact, it appears narrower than it really is). However, analysis of the bird in the Luneau video in light of images of known Pileated Woodpeckers confirms that a similar black trailing edge to the wing is discernable in some frames of the Luneau video (compare Figure 1B, frame 775 with Figure 1A, frame 366.7: the apparent plumage patterns are similar, and inconsistent with Ivory-billed Woodpecker). It is argued here that the hypothesis put forward in Sibley et al  is correct, and that the black trailing edge of the underwing of Pileated Woodpecker can indeed, due to flexion of the wings during the downstroke, be misinterpreted as the black leading edge and wingtips of the upperwing of an Ivory-billed Woodpecker.
3. Figure 3 shows that the plumage patterns shown by the Luneau bird, throughout several wingbeat cycles, are compatible with Pileated Woodpecker. The three plumage features described in Sibley et al  that are incompatible with Ivory-billed Woodpecker (black secondary feathers on upper surface of left wing, brighter white primary bases, and a black band curling round the wing tip) are seen consistently in the Luneau video and are recapitulated throughout the video of Pileated Woodpecker.
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