Some Common and Slaty-backed Gulls and a GWGU Hybrid?

Nial Moores, March 7th 2015

As we know, Korea is a great place for birding and for helping push the frontiers of knowledge on ecology, conservation and identification. There are many obviously exciting species here (from cranes to shorebirds, East Asian ducks, flycatchers and buntings) and also quite a few widespread taxa that can sometimes be a little too easy to take for granted, but which might well be of huge interest to birders elsewhere. This second group includes Slaty-backed Gull and “Kamchatka” Common Gull. Following several recent emails from points north, west and south in North America asking for (really!) specific details about these gulls, I decided to capture a few more images close to the office.

First up, Common Gulls…Our checklist lists two subspecies: the (generally) smaller, paler, neater, more quickly-maturing heinei and the larger kamtschatschensis, which in First-winter plumage is believed to be more heavily-marked on the underparts and to show a wider tail bar. Two birds when side-by-side seem to show these and a few other differences quite well (though it still seems best to wait for analysis being undertaken by Chris Gibbins and others before confidently labelling either of these to subspecies!).

comgulls1_rscomgulls2_rscomgulls3_rsFirst-winter Common Gulls Larus canus (with Black-tailed Gull Larus crassirostris), March 6th © Nial Moores

Lacking the right kind of camera gear (being a digiscoper with a compact digital camera), it is hard to capture images of the spread wing of any gull – here two less-than-sharp adult Common Gulls (the first shorter-billed and paler-saddled) showing the more or less “expected” upperwing pattern.

adcomgull1_rscomongullad_2_rsAdult Common Gulls Larus canus, March 6th © Nial Moores

Identification of adult Slaty-backed Gull is generally quite straightforward: here a view of the famous “string of pearls” (which, however, can also be shown by vegae, though usually less obviously).

sbguad_1Adult Slaty-backed Gull Larus schistisagus, March 6th © Nial Moores

 Identification of Second-winter Slaty-backed in late winter is also usually pretty straightforward too.

2wsbgu_rsSecond-winter Slaty-backed Gull Larus schistisagus, March 6th © Nial Moores

 However, First-winter Slaty-backeds (like vegae and many other large white-headed gull taxa) really do show tremendous individual variation. By March, a few birds are so bleached (and relatively featureless) that they sometimes can get misidentified as Glaucous Gulls by the unwary. The palest bird of the day, oddly, also showed an exceptional amount of pink on the bill.

sbsgu1_rs-pale-end“Pale-end” First-winter Slaty-backed Gull Larus schistisagus with atypical bill colour, March 6th © Nial Moores

 While other Slaty-backeds are obviously darker.

sbsgu_darker_rs“Dark-end” First-winter Slaty-backed Gull Larus schistisagus (with extreme end of greyish tones to the underparts),March 6th © Nial Moores

Looking at such birds, it is important to note that, with few exceptions, the extent of bleaching is more or less consistent within individuals: birds that are extremely bleached on the wing coverts and saddle tend strongly also to have bleached tertials, primaries, tail, rump and uppertail coverts. In contrast, birds that have well-marked “colored” saddles and scapulars tend strongly also to have rather darker primaries, though invariably blackish-brown rather than solid black.

This recent paper describes all the main features to look for in some detail. In short, things to look for in a standing First-winter Slaty-backed Gull in late winter include structure (strong-looking, with shortish primary projection and thick legs set well apart); a broad secondary “skirt” (in the image above, the dark line below the bleached-out wing coverts) and a big tertial step; plain-centred tertials with a broad pale fringe; bleached greater coverts (lacking much patterning, sometimes shading from dark to light); brown-toned primaries; and often stand-out dark spots or streaks on the scapulars.  Sometimes, some of the lower scapulars look grey-washed too. In most, the bill base is starting to pale, and the legs are clean pink. Preening birds and birds in flight show extensive white on the rump, and broken waves or speckles on the rump and uppertail coverts, contrasting well with the largely dark tail. Even dark-end First-winters tend to have much white showing in the rump (see below), and like paler birds they lack well-defined barring towards the “rear end” (especially by February or March).

Most First-winter Slaty-backed Gulls also lack obvious dark on the shins (“scutes”) or any obvious grey tones to the head, flanks or belly: those with extensive dark on the shins or grey on the head or underparts tend to show other unusual features (generally suggestive of hybridisation – or a misidentification!).

Despite actively searching for hybrids between Slaty-backed Gull and Vega or Glaucous-winged Gull, however, such birds seem to be genuinely very rare in Korea (though perhaps they are rather more usual in northern Japan?). As such, an odd-looking Glaucous-winged-like First-winter that joined a small group of Slaty-backeds provided an excellently interesting end to the afternoon…

hybrid1-rshybrid2-rsPresumed hybrid gull (rear) with same dark-end First-winter Slaty-backed Gull, March 6th © Nial Moores

hybrid3-rsPresumed hybrid gull (rear) with First-winter Slaty-backed Gulls, March 6th © Nial Moores

hybrid4-rsPresumed hybrid First-winter gull, March 6th © Nial Moores

Most features on this bird recall Glaucous-winged Gull. It shows quite pale-edged primaries (with nice big “V’s”); the underside of the primaries also lacks the well-defined dark edge typical of Slaty-backed and some other large gulls, and there is some barring across the vent, rump and uppertail coverts (though showing more white and less dark than most Glaucous-wingeds). Some of the closed wing also looks okay for Glaucous-winged, e.g. its more neatly-patterned greater coverts. However, the upperparts look too dark-speckled and patterned; the primaries seem a little dark; the grey tones in the upperparts seem too subtle and too dark (in many Glaucous-wingeds by March there are areas or patches of cleaner, paler grey); the bill is obviously paleing towards the base; and the jizz seems wrong for Glaucous-winged.  The legs look a little long (and bubble-gum pink rather than bruised-looking like many Asian Glaucous-wingeds); the secondary skirt seems rather long; the head looks (oddly) gentle; and the bill seems too delicate. Is this perhaps a Glaucous-winged x Slaty-backed Gull hybrid?

Or less likely, might it even be a Glaucous-winged x American taxon?

As always, informed comments (below or to my email) are warmly welcomed!



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