Black's Creek Reservoir, Ada County

25-27 August 2004

by Harry Krueger

[The following is taken from a post I made on IBLE on 8/31/2004 and a private e-mail I sent to Cliff Weisse of 8/30/2004, as well as a personal communication received from Phil Pickering of Oregon. The specimen of the Blacks Creek Reservoir bird resides in the Archive and Collection area of the World Center for Birds of Prey, Boise.]

LTJA bill

Bill: In a super-close-up...the length of the nail (from its tip to the back edge - essentially, the darker portion of the bill) is, by measurement, exactly equal to the rest of the upper mandible, even though at first glace, optically, the picture of the bird does not seem to support that fact. This is characteristic of Long-tailed Jaeger, whereas in Parasitic the nail is shorter (perhaps 1/3) of the upper mandible.

The gonydeal angle of the lower mandible is roughly at the same point as the back edge of the nail on the upper mandible, whereas in the Parasitic Jaeger it should be about half way.


Plumage pattern: The bird does not have any of the warm, golden, brownish hue that a juv. Parasitic would be expected to show, but rather is much more grayish, with whitish, narrow feather edgings (which characteristic is not present in Parasitic Jaeger).


Also, the heavily barred coverts and flanks are characteristic of Long-taiiled Jaeger rather than Parasitic.

Overall shape: overall impression was of a delicately and proportionately shaped bird when comparing head to body, whereas Parasitic Jaeger has always seemed small headed because it is a much "bulkier/chunkier" bodied bird. Again, although the pictures may not necessary show this characteristic to its best effect, the Blacks Creek Reservoir bird was quite "round headed."

LTJA Additionally, a PAJA should have conspicuous buff chevrons on the primaries, which this bird does not have.  

The underparts on the Blacks Creek bird seem to be much lighter than on the Long-tailed Jaeger at Island Park a few days later.

LTJA tail

When the bird did take flight, it was somewhat "tern-like" in flight, although the central tail feather extensions where not clearly noted, they did not look particularly pointed from underneath.

Note that the central two feathers are rounded and white tipped. This too is characteristic of Long-tailed Jaeger, while Parasitic is spiked or more pointed and not white tipped.

LTJA In the upper wing views, note that only the first two primary shafts are white (make sure you are looking at shafts, not feathers). In a Parasitic Jaeger (with which the Long-tailed Jaeger can most easily be confused), the first 3 to 5 primaries are white shafted.

Pickering's Comments:

It has the "cute" proportionately large-eyed, round-headed, slender-bodied look I find quite distinctive. As with the photos of the bird Cliff sent me [Island Park Long-tailed Jaeger of 8/28 through 9/3/2004], it also appears to have a pronounced concave area to the face in front of the eye, which at least in my experience appears to be a very good mark for Long-tailed Jaeger.

I do think it is a juvenile. The primary tips all look relatively unfrayed but also all fairly rounded, which I believe indicates a juvenile. Also, I would expect to see more substantial fraying to at least some feather tracts on an older bird in August. Relatively speaking this bird appears fairly fresh overall....At a quick look I also don't see anything about the patterning that suggests it's not a (pale-morph) juvenile [Long-tailed Jaeger].

Please note: Much of the above is also applicable to the Long-tailed Jaeger at Deer Flat NWR of 1-6 September, 2000.

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