GULLS smithsonianus
Western
Vega
Temp 4
Temp 5

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A POSSIBLE FIRST CYCLE SLATY-BACKED GULL IN CALIFORNIA

Here are photos of a bird in Half Moon Bay, California that could be a first cycle Slaty-backed Gull. The identification criteria for this species are far from being clear, and surely only the extreme individuals are identifiable here. The main problem, I had predetermined, was that Herring x Glaucous-winged could potentially create a Slaty-backed Gull like bird. Upon finding this bird, an entirely different issue came up, that of Western Gull, or perhaps Western x Glaucous-winged. Some context is important, the flock this bird was in was comprised of perhaps 6000 Western, Glaucous-winged, and their hybrids with smaller numbers of smithsonianus Herring, Thayer's, California and Mew gulls. There were ample Western Gulls to compare with, many right near the bird.

For photos of Slaty-backed Gulls, visit the Ujihara's Japanese Gulls Site.

Other Slaty-backed Gull candidates in Half Moon Bay, CA this year are here.

Steve Mlodinov is going to jump out and try to strangle me after seeing this! Just a couple of days ago I sent a rambling e-mail to him in a conversation about how the ID a first cycle Slaty-backed. I argued that the ID issue was with Glaucous-winged x Herring hybrids, based on what I have seen in photos from Japan. But I argued that we really do not know what Glaucous-winged x Herring hybrids look like, since we only identify them based on logical assumptions, but we really do not know that this is what they are. As such, we can't really know the variability of these presumed hybrids, it becomes a game of logic and assumption (even if the results are right!). But when this bird came through my binos, the structure was what got me, and not only did it not look like any possible Glaucous-winged x Herring hybrid I have "identified" but the real ID contender was Western Gull...not as I had expected. So this is a public apology to Steve, for being so closed minded on the subject.

First cycle.

Feb 2005 bird.

possible Slaty-backed Gull. Feb 8, 2005. Half Moon Bay, SM, California. A. Jaramillo

The gull stood out from Western Gulls in its structure. It was a large and bulky gull, certainly larger than Western Gull in mass. However it was noticeably short-legged, and it has a deep "keel" like a goose. This keeled appearance appears to be common in photos I have looked at of Slaty-backed Gulls. Slaty-backed appears to be a long-necked, but very bulky bird, with body circumference centered around the area near the legs. Western Gull, and smithsonianus Herring are much less deep bellied, looking slimmer altogether. This bird also had a relatively parallel sided bill, with little expansion at the gonys, certainly less so than on nearby Western Gulls of similar age, and suggesting smithsonianus Herring Gull more than any other species. Apart from structure, note the crisp streaking on the head, and the concentration of streaking around the eye. Western Gulls are much more evenly dark on the face, and do not tend to show this "mascara" look. A bit on the moult stage. Compared to approximately 50 Westerns of similar age noticed today the wing is fresher on this bird, including the tertials, and more juvenal feathers are retained on the scapulars. Western Gulls in early February do not show any juvenal scapulars, as Westerns moult early. Other points that stand out compared to similar age Western Gull:

- Fresh primaries that are dark brown, rather than black. Crisp white fringes on primaries.

- Lack of "skirt" suggesting a narrower wing than Western. Western Gulls usually show the secondary tips, or a large part of the secondaries on the folded wing.

- Greater coverts that are very uniform. Western shows well banded greater coverts.

PHOTO2 shows the bird's bill shape well. The bill lacks the well pronounced blob-tip of Western Gull. Also noteworthy is that the bill is quite short on this bird. The dark mascara look is obvious, as are the crisp, well formed streaks, particularly on the crown. In good light in the scope the eye could be seen to be pale brown, contrasting with the black pupil. The eye colour suggested that this will be a pale eyed bird once it ages. Eye colour is not an important feature, as Westerns can be pale or dark-eyed. The tertials show a good pale crescent, most Western Gulls are worn at this time of year, such that the tertial crescent is not all that obvious. The dark tail can be seen in this view, a feature common to Slaty-backed, as well as the other ID contenters.

PHOTO3 shows the the entirely dark tail, again typical of smithsonianus, Slaty-backed, Western, and Western x Glaucous-winged hybrids. The rump and uppertail coverts are whitish with dark speckles. The rump and uppertail covers are the whitest part of a first cycle Western Gull, and resemble the pattern on this bird, however the whitish rump and lower back area contrast more strongly with the darker scapulars and mantle on a Western than on this bid.

I do not have a shot of the spread wing, and I did not at any time see the underwing. However the spread wing from above showed distinctly paler inner primaries, extending a ways out on the wing although the exact details could not be determined in the views I had. Western Gulls of this age do not show a clear inner primary window. The inner primaries are paler than the outer ones, but not by much.

PHOTO4 illustrates the structure of this bird well. The first photo shows an odd bulging crop, giving a confusing impression of the bird. This photo more adequately shows the look of this individual. The body is stocky and large, very deep at the belly, giving a goose- like keeled look. The neck is thick at the base and long with a rounder head than on the typically more square-headed Western Gull. The bill is black and parallel sided, with little expansion at the gonys. The primaries are blackish, but not quite black, and they are longer than on Western Gull. The legs are short, when compared to Western. Note also the step pattern in darkness going from primaries to coverts. The primaries are blackish, the tertials are brown, and the coverts are paler brown. On Western Gull, the tertials are usually almost as blackish as the primaries. The skirt of secondaries is barely visible here, while on a Western Gull that was drooping wings, as this bird appears to be doing, the secondaries would be obvious. This is due to the Western's wider wing.

A word about Western x Glaucous-winged hybrids. These birds are common in winter in this part of California, and come from the hybrid zone centered around the Puget Sound area of Washington. The birds from this hybrid zone can look like anything intermediate between a Western and a Glaucous-winged Gull. The paler birds are much more Glaucous-winged Gull like of course, and show distinctly pale primaries. The darker birds are like Western Gulls, and show dark primaries. The darkest birds which are clearly hybrids, in other words separable in the field from Western Gulls, are very uniform in plumage. Their primaries can be blackish, but the body is just dark brownish-grey with little patterning whatsoever. The lack of clear pattern appears to be a feature gained from the Glaucous-winged genes. These birds are also washed greyish-brown on the neck and head, and are very uniformly dark. The mix shown by this possible Slaty-backed is much too well patterned on the coverts, scapulars, head and neck to fit this darkest extreme hybrid. I should snap a few photos of these hybrids to make this point more clearly.

PHOTO5 shows the bird sleeping. Perhaps one of the most interesting bits to focus on is the bird in the foreground which is a first cycle Western Gull. This Western is typical. It shows the very evenly sooty head colour, in particular the entire mask (lores to ear coverts) is evenly dark and looks largely unstreaked. Also typical is the lack of clear streaks on the crown, although some Westerns show this, and the pale surround to the ear coverts. This paler post-auricular crescent is characteristic of Western Gulls. Also note the Westerns first basic mantle feathers which are dark grey-brown, with buffy fringes. The Slaty-backed candidate is retaining much of the juvenal upperparts, but new feathers appear to lack dark centers, and are more evenly pale grey-brown.

PHOTO6 shows shows another view of the bird. Not much to add here, again look at the structure, pot-belly, stocky, but long-winged. As well look at the greater covert pattern, inner ones with banding, outer ones pale and largely unmarked. Check out those short legs.

WESTERN GULL 1 This image of a first cycle Western Gull shows a well streaked individual, at the more streaked end of the continuum. Even so, it lacks the crisply streaked look of the possible Slaty-backed, particularly on the evenly grey-brown mask. Rather than a dark "mascara" look of dark around the eyes, Westerns of this age look entirely dark masked, "bandit masked". A few structural features to look at 1) longer legs of this bird 2) even at this age, keeping in mind that bill size increases as immatures age, the bill can be seen to widen out a the tip 3) skirt of secondaries showing below the greater coverts 4) shorter wing. From a plumage state perspective, note that the tertial crescent is nearly worn off (photo taken the same day as the possible Slaty-backed), and the scapulars and mantle are pretty well all of a newer generation than the wings. Western Gulls, being early breeders, moderately sedentary, and low latitude breeders begin moulting out of their juvenal upperparts early in life. This individual also shows the typical upperpart pattern of dark centters and paler fringes. Note the banded, and sometimes striped, look of the greater coverts, out to the outers.

WESTERN GULL 2 Shows another first cycle bird, this one being a more evenly coloured individual. The bill is blob-tipped, almost droopy, and this is typical. This individual is a longer- winged one and shows the typical body shape, being thickset, but more cylindrical than the possible Slaty-backed Gull. The length of the legs above the joint is greater than that shown by the Slaty candidate. Again, the tertial crescent is much more worn, and the greater coverts are well banded. A secondary "skirt" can be seen. This late January bird retains only a few juvenal scapus, otherwise they are grey with dark centers. The head is evenly coloured, and this bird shows the more typical brown and unstreaked wash to the neck, breast and head. Again, that pale post-auricular crescent can be seen.