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Jaegers and skuas are a notoriously difficult group of birds to identify. Individual variation is extreme in this group, and in many cases, a single photograph is but a starting point for identification. Nonetheless, a single photo may prove sufficient to make a positive ID-and I would argue that this one is.
Let's start by aging this jaeger/skua. The fresh, crisp edging to the wing coverts is found only in juvenile jaegers and skuas. Thus we can confine out discussion to juveniles. The next step is to determine if we are dealing with one of the larger skuas-those birds formerly classified in the genus Catharacta (in this case limited by the quiz rules to Great Skua and South Polar Skua), or to one of the jaegers (Parasitic, Long-tailed, or Pomarine Jaeger).
At a glance, out bird looks rather skua-like, with a large, heavy body and a rather short tail. But the head of our bird is quite small. Even more importantly, out bird's bill is relatively thin compared to the thick-billed skuas. Juvenile South Polar Skuas would not appear as warm-colored as our bird. Juvenile Great Skuas, while sharing our bird's rusty-edged wing-coverts, would not show conspicuous rufous tips to the primaries. Furthermore, a juvenile Great Skua would have a darker head, making the bird appear hooded.
So we are left with the three jaegers. While jaegers are exceedingly variable, a jaeger that that appears this rusty on the upperparts is usually a Parasitic. Still, we want to be more certain than that, so let's look for additional clues.
Structually, this bird appears rather plump-bodied and small-headed, with a triangular-shaped head. The bill appears narrow, with a bit less than the distal third appearing dark tipped. Long-taileds typically show about half of the distal tip dark, while Pomarines typically have thicker bills (Olsen and Larsson).
As we already noted, the primary tips are distinctly rufous-tipped, another strong point in favor of Parasitic Jaeger. Pomarine Jaegers only rarely have pale tips to the primaries (Olsen and Larsson). Although Long-tailed Jaegers may have paler trips to the primaries, they should not be so rufous. The undertail coverts on our bird are distinctly less barred than on Pomarine or Long-tailed Skua.
The overall coloration, structure, bill size and coloration, rufous tips to the primaries, and minimal barring on the undertail coverts are enough for us to be sure this bird is a Parasitic Jaeger.
This juvenile Parasitic Jaeger was photographed by the author on 22 August 2003 at Clam Lagoon, Adak Island, Alaska.
For a more exhaustive treatment on the complexities of this difficult group, see Olsen and Larsson available from ABA Sales.
The following people (listed by submission date beginning with the earliest) submitted correct answers for the September Bird Photo Quiz—Parasitic Jaeger:
The graph below shows that the highest number of answers submitted was for Parasitic Jaeger (51), followed by Pomarine Jaeger (5). We received a number of answers that did not conform to the ABA Checklist format. Please note that answers must consist simply of the Common or English name exactly as it appears in the ABA Checklist.
The photo and answer for this quiz were supplied by Chris Wood.