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Kirby Jaeger Identified as Pomarine

Jaeger_LDB_061_cr.jpg

For those of you who are following the discussions on Texbirds on the Kirby Lake Jaeger, Martin Reid posted his views regarding its identity. Below are his comments along with the photos submitted to highlight his conclusions. The full-sized photos can be found at Flickr. Lorie Black’s photo (above) is also used to highlight some of Martin’s points. The jaeger was at the lake again this Monday morning.

Dear all,
I’ve been asked to comment on the identity of the Kirby Lake Jaeger. I believe it is a Pomarine, and I list here the features that lead me to this conclusion (from most important/reliable to least important/reliable). It should be kept in mind that there is probably no one single 100% foolproof feature to separate an “average” intermediate/dark Pomarine (POJA) from a similar Parasitic (PAJA), and thus an ID based on a large number of features is likely to be more accurate. I need to disclose that my personal experience with jaegers is not huge: Since moving to Texas almost 17 years ago I’ve seen 50 – 60 POJAs, 25 – 30 PAJAs, and 3 LTJAs. In 2006 I was based on Alaska’s North Slope for six weeks in late August and September where I had daily encounters with numerous breeding (and a
few juvenile) POJAs and much smaller numbers of PAJAs. Back in my native Great Britain many years ago I did a lot of land-based seawatching and visited PAJA colonies that provided extensive experience with POJA and PAJA (hundreds of each;- maybe more than a thousand for PAJA). My conclusions and my comments below are based on my experience plus the content of the following references: “Field Identification of the smaller skuas (= jaegers)” By Klaus Malling Olsen and Lars Jonsson as published in BRITISH BIRDS Volme 82 number 4 (April 1989). “Skuas and Jaegers: A guide to the Skuas and Jaegers of the World” By K. M. Olsen and Hans Larsson. “Advanced Birding” by Kenn Kaufman. “The Sibley Guide to Birds” by David Sibley.

First it needs to be aged, if possible; luckily this individual has all the hallmark features of a juvenile (fresh plumage with much pale edging/tipping on the upperparts, all feathers appearing the same age; legs (not feet) completely blue-gray; central tail feathers very short, etc.) To save repetition all the following comments (unless specified) refer only to juveniles:

Reasons why it is a POJA:

Jaeger_scapulars

The primary tips have no pale markings at the tip. One image (IMG_5116 above) seems to show a tiny white dot on two of the right wing primaries – but on all other images including those that are closer and sharper than IMG_5116 there is no sign of any pale primary tips. PAJA differs from almost all POJAs (and LTJAs) in that the primary tips have obvious pale tips – usually formed by a pair of short, thickish, cleanly-demarcated pale lines on each side of the tip that almost meet at the very apex of the wing. The only time a PAJA would have the Kirby bird’s lack of pale markings to the primary tips is if it were extremely worn, and such wear would also be evident in reduced/absent pale tipping to the wing coverts. On this bird the pale tipping to the wing coverts is thick and fresh-looking, so it is hard to imagine that the primaries ever had similar pale tips.

Jaeger_tail

The Central pair of tail feathers are very short – barely extending beyond the rest of the tail – and the right-side central tail feather (the left-side one looks damaged and unreliable concerning shape) appears to have a broad, shallowly-curving tip that does not end in a point. POJA has the shortest-looking central tail feathers at this age, often presenting a rather squared-off tail in all but the most ideal angles of view (and as shown in a number of the flight pics of this individual). Although hard to discern in many instances, PAJA should show a short but obvious fine point at the tip of these feathers.

Jaeger_head_posture_IMG_5162

The shape and pattern of the head: PAJA typically has a rather distinctive crown shape, with a high-point above or ahead of the eye that forms a domed look; in contrast POJA has a more-squared head shape, and if there is a peak it is a very gentle one behind the eye, creating a blocky look. The Kirby individual seems clearly to have this latter shape. POJA has a rather smooth head pattern without any longitudinal streaking in the nape (but sometimes with a suggestion of dark scaling) and throat, and with the darkest part of the head being on/above the loral area. PAJA invariably has some diffuse but obvious longitudinally-aligned streaking on the nape, plus finer but more distinct streaking on the throat, and with the darkest part of the head being the center of the crown, creating a capped effect that – on all but very palest-headed individuals – consistently creates a soft, capped look in flight accentuated by the contrastingly pale nape. I see no signs of this latter pattern in the Kirby bird (whose head does not paler than the body, at rest), instead I see a fairly classic POJA pattern, with no sign of streaking on the throat or nape.

Jaeger-head

NOTE: photo IMG_4939 does suggest a dark cap – but it is subject to strong low-angle lighting that I feel is presenting an inaccurate view – compare it to all the images taken the next day in neutral lighting.

Jaeger_rump_IMG_5117

Rump and uppertail covert pattern: PAJA typically have relatively low contrast in this area, as the paler elements are typically the same tone as that of the head (or slightly darker), and the pattern of darker and lighter bars does not line up to create fairly straight latitudinal barring. Only pale-headed PAJAs have a contrastingly pale rump/utc area. POJA typically has the pale elements in the rump/utcs lighter (sometimes strikingly-so) than the head, and has the marks line up to form latitudinal bars that are straightish or with a slight scalloping. The Kirby bird is slightly atypical of POJA in that the pale elements of the rump/utcs ar barely any lighter-toned than the head – but the pattern of this area is typical of POJA (and not of PAJA) in that the dark and pale marks are for the most part uniformly thick an aligned into almost-straight lines, with obvious contrast.

Jaeger_upper_IMG_5182

The outer lesser secondary coverts of the upper/leading edge of the wing: PAJAs of this color-type (i.e. not dark-morph, pale-morph, or “cold-gray”-morph) have very extensive pale rusty tipping to these feathers, forming a palish rusty band on the leading edge of the wing (often merging into the similar tipping/edging on the median secondary coverts). POJA does not have this strong pale tipping to the lesser secondary coverts, thus the leading edge of the upper wing is darkish. Some POJAs have quite extensive pale rusty-cream edging to the median coverts and on such birds, there is usually a contrast on the open wing between the darker outer lesser coverts and the paler medians (may include some of the lowermost lessers). The Kirby individual has well-edged median upperwing coverts with darker lesser coverts, as on POJA.

Jaeger_coverts_IMG_5212

Pattern of upper median and greater wing coverts: PAJA typically has, in addition to the crisp pale marks at the tip of each of these feathers, one or more obvious pale marks either on the edge or internally about half-way back from the tip of the feather (sometimes there is a complete pale edge to the outer side of each feather). POJA typically lacks these marks, and at most has a very small number of such marks on the innermost greater coverts. The Kirby bird has three tiny such marks on the inner greater coverts and none elsewhere, better-matching POJA rather than PAJA (although some PAJA can be poorly-marked.)

Jaeger_foot_color_IMG_4944

Foot color: POJA differs from PAJA (and LTJA) in that, while the leg and “ankle” are pale blue-gray, almost the entire foot is black, while the smaller species have varying amounts of blue-gray extending from the ankle onto the sides of the foot at its base. The Kirby bird appears to have a virtually all-black foot with no pale gray at the base, either side of the “ankle”.

Jaeger_bill_LDB_061

Bill shape and size: Often hard to judge, one might argue that this individual is within the upper limit for PAJA, but only a small minority of PAJAs might appear like this, while it is typical for POJA. Also note the rather large hook for a bird of this age (where the bill may not yet be fully-grown).

Jaeger_chasing_Yellowlegs_IMG_5274

Overall size: impossible to determine from all but one of the photos; the one pic with a yellowlegs close-by gives me the impression of a really large jaeger, and the observers saw it close to Ring-billed Gulls and said “this jaeger is easily the size of the Ring-bills that it was harassing”. Per Kaufman, POJA is about the size of a Ring-billed Gull or Heerman’s Gull, while PAJA is the size of a Laughing Gull.

Jaeger_LDB_061_pale_base_primary_coverts

The bases of the underwing greater primary coverts: The oft-touted pale bases of these feathers that is characteristic of POJA is somewhat variable. A minority of PAJAs can have a contrasting pale area that matches the typical pattern of POJA, while a minority of POJAs can lack strong contrast at this location. This is perhaps the only feature of the Kirby bird that could be viewed as being non-typical for POJA – but the pattern falls well within the limits for POJA.

Taking all the above into account, I feel that the evidence is overwhelmingly in favor of Pomarine… but I am open to a well-argued alternative analysis!

Regards,
Martin

One Response to “Kirby Jaeger Identified as Pomarine”

  1. Bera Says:

    Leon and I enjoyed watching the Pomarine Jaeger yesterday afternoon (Tues.) at about 5:00 on the west side of the lake. It really put on a show for us and came very close. Such a strange phenomenon for it to be so far out of its usual habitat. We were so excited that we were able to find it!