Posted by Phil Pickering (184.108.40.206) on October 24, 2001 at 22:25:02:
The comments of others, and my own first impressions that it is useless to try to detect proportional differences in single-bird Long-tailed/Parasitic photos has left me feeling challenged to see how similar or different these two species really are in shape (too much free time, I guess). The majority of my current birding time is spent doing seawatches, occasionally struggling with possible (streamer-free) Long-tailed Jaegers seen in silhouette, so this is not an entirely meaningless obsession.
Looking at jaeger pictures I can see how the hand/arm ratio would most often be useless in photos even if there is an actual difference, because if the wing is bent, anything but a direct view from above or below will result in the hand and arm being at different angles to the camera, leaving one more foreshortened than the other. Depending on posture, particularly the position in the wingstroke cycle, the wingbase may also be at a different angle to the camera than the tail, so their apparent ratio is likely most often unreliable in photos despite it being a valuable enough field mark to merit its mention in Sibley. I believe the more rounded posterior margin of the hand in Parasitic is also a valid mark, but it is probably only useful for birds studied in the field for an extended period, as its appearance in any one photo is entirely dependant on both posture and angle to the camera.
There is another previously unmentioned "jizz" feature that may be somewhat less ambiguous, and perhaps of more use than other proportional differences as a supporting mark in photos. To me, despite the foreshortened view, the head, neck, and shoulders seem to project quite substantially and broadly beyond the wings on this bird. For whatever it's worth, factoring in the angle of view, my impression is that the head should look noticeably smaller and the neck should appear shorter if this were a Long-tailed. Even in direct profile, Long-tailed in flight most often seem somewhat small-headed and short-necked, while Parasitic have a noticeably longer neck, broader shoulders, and seem to have a proportionately larger head than Long-tailed. In photos at least, the appearance of the "front end" seems to be one of the more consistent and easily detected proportional differences. It's just a question of how accurate a feel you can get for it in a photo of a bird flying away from you - although I would think the tendency would be for the front end of a Parasitic to look proportionately more like a Long-tailed at this angle, rather than the other way around.
I also get the impression that there is a fairly pronounced bulge apparent high on the upper breast of this bird. I think the bird might very well look too barrel-chested for Long-tailed if seen in side profile (although this is quite hard to judge at this angle). Parasitic often show a quite pronounced bulge high on the breast, and sometimes (not always) a comparatively steep angle between the top of the breast and the neck is obvious, perhaps in part because the neck is longer in Parasitic. In flight, Long-tailed do sometimes show somewhat of a projecting breast, but it is most often more gently rounded, without such a pronounced bulged look, and most often with a fairly shallow angle between the top of the breast and neck. In fact, my impression is that the steeper the angle at the top of the breast becomes, the more the shorter neck of Long-tailed seems to disappear. Most often, the upper breast in Long-tailed appears comparatively quite flat. While this may be of suspect value in an angled photo like this, I think the more prominent, often bulged-looking upper breast of Parasitic is likely one of the more useful supporting marks in the field. This difference seems to be underemphasized to some extent in field guides, and is often quite apparent in photos showing a side profile.
Overall, in my view this photo might be more deceiving than is easily realized - I suspect that the wings are broader, the body longer, and the bill longer and thicker than they "feel" here due to the angle of view, likely combined with the foreshortening effect of telephoto mentioned by Bjorn. Looking at other photos has also reinforced the impression that the rump appears somewhat wide for Long-tailed. Individually, these proportional features are admittedly ambiguous and subject to personal judgment, but I am starting to get a strong feeling that their sum is worth more than their individual values, and if it comes down to Rivera vs. Sasaki would it really have mattered?
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