Posted by Phil Pickering (208.46.229.86) on December 30, 2001 at 01:43:50:
I recently picked up Olsen and Larrson's Skuas and Jaegers, so now I can really pretend like I know what I'm talking about. Since this jaeger is still a "mystery", I thought I'd offer my new perspective, for whatever it's worth.
After studying the book and every internet photo of Parasitic or Long-tailed I could find, I have to go back and dump my previous posts on this bird, and do another flip-flop (emphasis on flop). It's clear that some of my initial impressions about this bird, and also preconceptions about jaegers in general weren't very accurately formed (see scaup). I now feel reasonably confident that the mystery bird is a Long-tailed, based mainly on distinctive shape characteristics. I think Long-tailed should now be the first option to consider anyway, since the bird was near a Long-tailed colony and no Parasitics were seen in the area. This fact certainly isn't diagnostic, but I think its importance is easy to underestimate. Dave Fix likes to talk about some Occam dude who plays with sharp objects...
Like some others, my first impression of this bird was that it was somewhat slight for a Parasitic, but I got completely sidetracked in trying to read too much into the effects of foreshortening. I now think the bird looks slight because IT IS, and not due to camera angle or telephoto compression. I think Tony Leukering mentioned the thinness of the body. Indeed, this bird is VERY thin side to side for a Parasitic, all the way from the rump to the neck. Telephoto "squashing" shouldn't affect this perspective. The head also appears remarkably small and blunt for even a compressed Parasitic. Parasitics seem to most typically have more of a pointed look to the head than this bird, particularly when viewed from beneath. The wings also really do seem quite narrow for a Parasitic, most evident at the wing base. Parasitics seem to most often show more evident taper to the wings as a whole at most angles. All things considered, there does not appear to be enough bird there for the typical size range of Parasitic I've seen in photos. The bird as a whole seems compact in a way that could not be due entirely to photographic distortion, if it is at all.
Given these impressions, I must say that the bill is very likely (nearly) as short as it looks, which is perhaps even on the short end for Long-tailed. O and L indicate that there is some overlap in the range of bill length between the two species, but in photos the vast majority of Parasitics appear longer-billed than this bird, even at odd angles. The dark area formed by the eye and perhaps a small adjacent dark mark seems quite large in proportion to the head. This seems to be a much more typical look for Long-tailed, and I think adds at least a hint of perspective of the bird's actual size.
In photos, I've also noticed that Long-tailed sometimes (not always) have a strikingly contoured look to the front of the head. The upper face can appear quite pinched, with pronounced depressions in front of the eyes. The eye socket or eyebrow ridge can also seem somewhat bulged in some Long-tailed. I wondered whether this might not just be an artifact of condition of the head feathering or other cause that wasn't necessarily distinctive for Long-tailed. However, I have yet to see this look any more than hinted at in photos of Parasitic. The upper face of Parasitic seems consistently more evenly tapered. Some seem to show slight depressions in front of the eyes, but seemingly never to the extreme extent shown by some Long-tailed. Perhaps there is an actual difference in skull structure that is more apparent when the head feathering is flattened or worn, or perhaps this represents a male/female structural difference that is more evident in the smaller, blunter skull of Long-tailed. Whatever the cause of this look in Long-tailed, to me it seems unlikely to be a coincidence that the mystery bird has it, looking distinctly pinched in front of the eye. This could be off base, but I think it's at least worthy of further study. Perhaps it's distinctive enough to be diagnostic by itself for some Long-tailed, including the mystery bird.
As for the R1 shape, I'm not sure if it means anything or not. The amount of taper does not necessarily seem outside the range of either species in post-juvenile generations as far as I can tell. It also doesn't necessarily seem unusual for either species to show R2s projecting beyond the outer rects. This probably has something to do with wear/molt, maturing feather shape, or perhaps a combination of the two. This bird could have dropped an R2 prematurely, and is growing the more mature next generation feather ahead of schedule in its place. The R2s do seem a bit longer and rounder-tipped than the outer rects, even in adults. This could be because they have evolved to partially support the long R1s, and also perhaps because the R2s are less prone to the tip wearing off than the outer rects, because the shaft is relatively centered in the R2s and progressively less centered in the outer rects.
So I haven't been able to determine if there's really anything distinctive about the tail, but I did note one photograph in O and L (figure 135, pg 166 adult winter Long-tailed) that has a tail that appears more like the mystery bird's than in any photo I've seen. This bird has (both) similarly projecting R2s, and short R1s with tapered bases and straighter sharp terminal barbs. The R1s could certainly be growing on an adult in winter, which suggests a possible parallel that the R1s are still growing on the mystery bird, and that the sharp terminal barbs on more tapered bases represents a stage of R1 growth. O and L indicate that the R1s could still be growing on a 1st-summer bird in June, since the first post-juvenile molt is typically later than subsequent complete molts, often extending into the bird's first summer.
I think another minor supporting mark for post-juvenile Long-tailed might be the width of the dark undertail band. Parasitics seem to more often show a narrower dark terminal band, and more white at the undertail base, although this is quite variable.
Reading O and L gave me the impression that tail molt in jaegers isn't completely understood. The intro suggests that all jaegers molt R1s twice annually (R2-6s just once), but the individual species sections indicate that the evidence is that some Pomarines and perhaps ALL Parasitics only molt R1s once annually. There is also a statement about some adult Long-tailed growing "basic" R1s that are short and rounded as in juveniles, while one or two of the photos to which this statement seems to refer to illustrate the point seem to me to probably be of winter adults missing R1s and showing rounded projecting R2s. I assume there must be concrete evidence that Long-taileds can molt R1s twice annually, but the photos in the book seem to suggest that it's possible that not all of them do. It seems to me that there is such an extreme range of variation in the timing of R1 molt and growth in jaegers that it would be very hard to form really specific conclusions, unless it were possible to follow and sample individuals multiple times during the year.
As others have indicated, though, jaegers do typically have a complete molt in their first winter/spring, so that, unlike gulls, a 1st-summer bird may have comparatively fresh flight feathers. Thus, if the primaries are as fresh and unfrayed as they seem on the mystery bird, it wouldn't preclude it being a 1st-summer. O and L do postulate that a few, perhaps weak or malnourished individuals, can have a delayed or suspended molt, so that they largely retain their juvenile plumage well into their first summer or even second fall. These individuals can appear quite worn and faded, similar to 1st-summer gulls. It may be that some of these delayed/suspended molt birds are incapable of migrating, and are more likely to remain at our latitudes in summer, creating confusion about the timing and extent of the first post-juvenile molt (did for me, anyway).
The pale under-primary bases don't seem unreasonable for 1st-summer Long-tailed. There are photos of two different individual 1st-summer birds showing this in O and L, and I have seen at least 2 others, one a second calendar year bird photographed off the Oregon coast in October. I could be wrong about this, but my impression is that ALL jaegers have some white at the primary bases. It just doesn't typically extend out beyond the greater coverts in post-juvenile Long-tailed, while it does in the other two species.
As was indicated, it seems to be the patchy, pale, relatively unbarred areas in the under-primary coverts that might be more unusual for Long-tailed with second generation+ plumage. I HAVE seen 2 photos of juvenile Long-tailed, though, with under-primary coverts that looked quite similar to the mystery bird's, so I really have to wonder how firm a mark the pale areas can be for Parasitic. I can sort of make out the typical size/shape barring or spotting of a juvenile Long-tailed on the greater primary coverts anyway - it's just very faint. I considered the possibility that some or all of the primary coverts, and possibly other parts of the plumage, are actually still 1st-generation due to delayed molt, but I can't do more than speculate about that. Whatever the case, my impression is that there is just too much variability in appearance, and in timing of molt in both species to place too much emphasis on the appearance of the underwing in 1st-summer birds in photos.
Body plumage also seems to be quite variable, and probably too hard to judge at this angle and in this type of misleading lighting to give too much emphasis. However, I would say that the strong warm tint to the right P10 shaft and at the right primary bases suggests that the apparently similar-colored body and more importantly, the head (factoring in shadow) are whitish, perhaps even starkly white, which would be much better for Long-tailed. Hard to tell, though.
Forgive my rambling. Hope some of this is of help,
Phil