Monday, 19 February 2018

Choshi (4): Slaty-backed Gulls

I had a myriad reasons for wanting to visit Choshi: Get Thayer's Gull Larus thayeri; get Glaucous-winged Gull Larus glaucescens; see thousands of East Asian gulls close up; photograph the various cormorants; and so on and so on. In all of these departments, I would have to say that the trip was an overwhelmingly successful one! One of the key reasons for going there, though, was to take a closer look at Slaty-backed Gulls Larus schistisagus, specifically to try and figure out precisely how first-winters differ from same age Vega Gulls Larus vegae as the separation of these two had been causing me problems back in Taiwan. In this regard, I can only conclude that the trip was a failure as I left very much 'none the wiser' (if anything, even more confused). Starting on a positive note, though, adult Slaty-backed Gulls pose no identification problems whatsoever and are a marvellously attractive species, with their disgustingly dirty heads (in some) and striking string of pearls in flight. Attached are a few representative examples of this cracking gull.

Second- and third-winters, too, are a straightforward proposition to identify, as they are both pale-eyed and sport either some or a majority of adult-type dark greyish feathers in the mantle. Something about the structure (of many, but note already that the first individual above is uncharacteristically 'leggy'), 'mien', and general appearance is also quite distinctive.

The distinctive look of older Slaty-backed Gulls is also reflected in many first-winter birds, and the first few I encountered at Choshi are shown below. These are all obvious birds, with slightly drooping, short, thick, blackish bills, somewhat 'fluffed up' thickish/swollen-looking heads and necks, smudgy/dusky 'masks' around the eye, washed out/faded (and short) rear ends (tertials and primaries, either brownish or 'frosted-looking', creating a 'patchy' look to the upper primaries on the open wing), and similarly faded and patchy-looking plain tails with diffuse and faded-looking white terminal 'areas' rather than neatly defined narrow white fringes.

So far so good, but the seeds of subsequent problems are there to be seen in just these first few individuals. Firstly, the pattern of the scapulars is different on each bird, including 'juvenile-looking' ones with largely dark centres, 'juvenile-looking' ones with dark anchor marks distally, bleached and faded ones with indeterminate markings, and more 'adult-like' ones with blue-grey centres and fine shaft streaks. The greater coverts vary in a similar way, as do the tertials, with even the suggestion of Herring Gull-like 'piano key' patterns through the greater coverts and pale notches on the tertials of one bird. The only thing these four (perhaps five) individuals have in common is that they are all bleached or worn on their outer wings and tails. Elsewhere, there is variation, even in these first few 'obvious' first-winter Slaty-backed Gulls. Imagine my horror when I ventured further into the harbour to be greeted by this lot, all of which I presume to be Slaty-backed Gulls!

The last bird above is a confusing one as it has well-patterned scapulars and a 'piano key' pattern through its greater coverts. It is not especially bleached or worn, though it is bleached through its primaries and on its uppertail. This, together with the smudgy eye mask, suggest that it is Slaty-backed Gull rather than a Vega Gull. But what about these next few birds? The first is strongly patterned through its wing coverts but does have a smudgy face, plain centres to its scapulars, and rather plain tertials (so Slaty-backed Gull). The next has the 'oily-blue' cast of a Glaucous-winged Gull Larus glaucescens to it, but has a rather dark saddle and is very bleached and worn through its entire wing (so Slaty-backed Gull). The last one is perhaps the most confusing of all, as it seems not worn in the slightest and even appears rather long-winged. However, the broadly dark centres to the scapulars and the greater coverts would suggest that it cannot be Vega Gull. I saw one or two birds that looked like this one, and photographed one flying. These didn't look much like the other Slaty-backed Gulls around, even though I couldn't see what else they could be!

I left Choshi with far too few spread wing shots of this species, distracted by all the other (newer) gull species present there. Those that I did get show the same kind of variation as do the standing birds, leaving me wondering just what a 'typical' Slaty-backed Gull ought really to look like! The first bird below is an excellent approximation of my pre-visit Gestalt, with its extensively bleached upperparts except for a smudgy eye mask and a darker belly. The second bird is close, too, with its mostly black tail (though note some white at the base of R6), faded 'hand', and 'Venetian blind' effect through its outer primaries. The third bird is similar but has a more densely barred rump than I would have expected. The fourth bird is similar to the others, but note has Herring Gull-like inner primaries, with arrowhead tips on P1-4 and large pale rectangular areas in the outer webs of those feathers. I assume it must be Slaty-backed Gull due to its evenly dark greater coverts, as I do the fifth bird, which (unlike the others) has some worryingly complex barring on the base of R6!

So, 'as clear as mud' is the conclusion I have arrived at having failed miserably in my quest to isolate precisely how first-winter Slaty-backed Gulls differ from same age Vega Gulls. I assume all the birds pictured above to be the former, though cannot rule out having misidentified one or two. Whilst a large number of Slaty-backed Gulls are really quite obvious, others seem to be able to show one or more pro-Vega Gull features (arrowhead markings on the scapulars, piano key markings through the greater coverts, complex barring on the base of R6, fresh-looking plumage, and so on) in combination with pro-Slaty-backed Gull ones. I could have done with perhaps another day or two to look more closely at these things, but also wonder whether or not that might only have made matters worse! Above photos taken at Choshi Harbour, Chiba Prefecture 6-9/2/18.

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