Slaty-backed Gull (schistisagus) / オオセグロカモメ / 큰재갈매기

(last update: December 2015)

Slaty-backed Gull (schistisagus) sub-adult March

The immature birds on this page comprise two age classes: 3rd cycle (4CY March) birds and 4th cycle (5CY March) birds. Many birds can be aged correctly by combining adult-like and immature characteristics, however some retarded 4th cycle birds or advanced 3rd cycle birds are labelled "sub-adult".

3rd generation flight feathers in 3rd cycle birds. In general these are adult-like birds, with obvious immature markings. Ageing can be difficult at this age, especially between 3rd cycle and 4th cycle birds. Immature birds with extensive black in the tail and extensive brown wing-covert patches should normally also be in 3rd cycle. However, advanced 3rd cycle and retarded 4th cycle birds are extremely similar, and such birds are probably best labelled "sub-adult".

4th cycle birds are very adult-like, with single vestiges of immaturity, like dark centres on underwing primary coverts, dark centres in some secondaries or some black in the tail feathers. Remember that true adult Slaty-backed Gulls hardly develop black on the bill, hence birds that do show a black line on the bill are suspecious and should be checked for any immature characteristics (check white underwing on dark scalloped patterns!).

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Below you will find a description of Chapter 30. SLATY-BACKED GULL, as published in one of the best Gull publication: "Gulls of the Americas" by Steve Howell & Jon Dunn.

"we" in the text below refers to the original authors. If any errors occur in this text, please let me know and mail to marsmuusseatgmaildotcom.

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BACK TO PART 1: STATUS & DISTRIBUTION

BACK TO PART 2: FIELD IDENTIFICATION, SIMILAR SPECIES - ADULT CYCLE

BELOW: FIELD IDENTIFICATION, SIMILAR SPECIES - FIRST CYCLE

For Slaty-backed Gull, note overall size and structure, bill size and shape, two-tone pattern of outer primaries, relatively plain greater coverts.

Slaty-backed Gull (schistisagus) 1st cycle (1CY), October 20 2011, Chiba Prefecture, Japan. Picture: Seichoudoku. Juvenile plumage, several upper scaps replaced for 2nd gen feathers.	Slaty-backed Gull (schistisagus) 1st cycle, December 27 2009 - January 04 2010, Choshi, Japan. Picture: Chris Gibbins. Wholly dark tail and a well barred rump and upper tail coverts, uniform primaries. Plain tertials and simply patterned second generation scapulars. The underwing coverts are very uniform, softly patterned with brown.

VEGA GULL less thickset with narrower and relatively longer wings, less stout bill typically dull pinkish at base; some birds may overlap in size an d overall shape with Slaty-backed. Plumage generally has more-contrasting patterning, notably, on greater coverts; tail base whiter with narrower and blacker distal band; A1 scapulars neatly marked with bars and anchor patterns; outer primaries darker on inner webs, so spread wingtips lack pale tongues of Slaty-backed. Bleached first-summer Vega Gulls often have blacker tertials; note scapular and tail patterns.

Vega Gull (vegae) 1st cycle, January 31 2010, Choshi, Japan (Gullog). Piano key pattern on GC but hardly any notching on TT. Scap moult: 50-75%.	Vega Gull (vegae) 1st cycle, January 31 2010, Choshi, Japan (Gullog). Atypical bird in a European context, with no notching on tertials (and tertials darkest part of bird together with visible primaries like in cachinnans) and very simple pattern on 2nd gen scaps.
Vega Gull (vegae) 1st cycle, January 25 2011, Choshi, Japan (Terry Townshend). Dark secondary bar. Pale panel on inner primaries reaching outer secondaries S1-S3 and still innerweb of P6 largely pale.	Vega Gull (vegae) 1st cycle, January 31 2010, Choshi, Japan (Gullog). Very similar to European Herring Gull. Piano key pattern on GC and deep notching on TT. Scap moult: 75-99%.
Vega Gull (vegae) 1st cycle, January 27 2011, Choshi, Japan (Terry Townshend). Very similar to European Herring Gull. 1st gen flight feathers.	Vega Gull (vegae) 1st cycle, January 31 2010, Choshi, Japan (Gullog). Pale grey-brown base colour of scaps suggest these feathers were replaced quite recently, and still not bleached. Scap moult: 100%.

AMERICAN HERRING GULL (widespread) averages less thickset with narrower and relatively longer wings, averages less stout bill that is typically dull pinkish at base. Plumage generally darker and browner with bolder pale patterning, strongly barred tail coverts; outer primaries darker on inner webs, so spread wingtips lack pale tongues of Slaty-backed.

American Herring Gull (smithsonianus) juvenile X50 August 30 2012, Gloucester, MA (Keith Mueller). Juvenile fresh dark plumage, but already single feathers in upper scapulars seem to be missing. Note overall dark brown aspect, bill size and shape, and dull pinkish on bill base.	American Herring Gull (smithsonianus) 1st cycle, October 08 2006, Newfoundland (Bruce Mactavish). Juvenile plumage, no scaps replaced by early-Ocober. Uniform under-parts and neck, dark tail, bold pattern on undertail-coverts. Thin white fringes on primary tips.
American Herring Gull (smithsonianus) 1st cycle, February 09 2008, Dryden, NY (Mike). Brown aspect. Fresh feathers (grey) on hindneck, side of the neck and flank. Crown and ear-coverts replaced as well, throat worn whitish 1st gen feathers. Pale bill base.	American Herring Gull (smithsonianus) 1st cycle 1106-13848 December 07 2013, New Buffalo, MI (Amar Ayyash). All scaps replaced. Much of brown aspect replaced in neck, side of breast and flanck, now cold greyish.
American Herring Gull smithsonianus, juvenile, September 22 2012, Plum Island, MA (Suzanne Sullivan). Note heavily barred rump and uppertail-coverts, barely contrasting with back; also, note almost entirely dark rectrices with contrastingly pale shaft and narrow 'barred' strip along outermost edge.	American Herring Gull (smithsonianus) 1st cycle, March 02 2013, Circle Beach, CT (Keith Mueller). Some dark pigmented birds have a poor contrasting paler window on the inner primaries.
American Herring Gull smithsonianus, juvenile, August 25 2010, St. John's, Newfoundland, Canada (Dave Brown). Note pale window on inner primaries, dark base to greater coverts, densely barred rump and undertail and very limited vermiculation along edges of outermost rectrices.	American Herring Gull (smithsonianus) 1st cycle 1106-27465 December 07 2013, New Buffalo, MI (Amar Ayyash). Note paler panel on inner primaries, poor contrast between outer and inner webs on blackish outer primaries, mostly blackish tail and note bill size and shape.

WESTERN GULL averages more thickset with more bulbous-tipped bill, plumage darker and browner overall, darker upperwings lack pale panel on inner to middle primaries, tail more solidly blackish.

Western Gull occidentalis 1st cycle (1CY), December 1985, California, USA. Picture: Alex Abela. Coverts and tertials still juvenile, some scaps replaced.	Western Gull occidentalis 1st cycle (2CY), February 1989, California, USA. Picture: Alex Abela. Note moult in head feathers, neck, side of breast and flank, new feathers are cold grey, contrasting with old bleached pale brown juvenile feathers.

THAYER’S GULL (w. N. America) smaller and more lightly built with narrower wings and smaller, less stout bill, shorter legs. Upperparts overall more brightly and boldly patterned. Wing and tail patterns similar to Slaty-backed but tail browner, and dark/light contrast on outer primaries averages stronger at shaft.

Thayer's Gull 1st cycle (2CY), mid-January 2014, central California, US (Amar Ayyash).	Thayer's Gull 1st cycle (2CY), mid-January 2014, central California, US (Amar Ayyash).
Thayer's Gull first cycle, January 28 2012, Pacific County, WA (Charlie Wright). Few scapulars replaced for 2nd generation feathers.	Thayer's Gull 1st cycle, February 16 2010, Winthrop Harbor, IL (Erik Bruhnke). Note pale windows on inner primaries which continue to the inner webs of the outer primaries. Also note the lack of dark pigmentation on the inner web tips of P1-P4, with just a bit of faint darker on P4.

HYBRID GLAUCOUS-WINGED GULL X AMERICAN HERRING GULL = "COOK INLET GULL" (w. N. America). Slaty-backed upperwing typically has relatively plain and pale greater coverts, contrastingly pale inner primaries, and more distinct inner web/outer web contrast on the outer primaries (at least in fresh plumage), but these features may be matched on hybrids; deep pink legs of some Slaty-backeds may be outside range of hybrids. Field characters need study.

Cook-Inlet Gull (glaucescens x smithsonianus) 1st cycle, January 18 2010, Morro Bay, CA (Amar Ayyash). The white edges to the primaries may suggest Thayer's but the size of this individual is much more telling. The upperparts are a bit more frosty than Thayer's and the bill is that of Herring - certainly not Glaucous-winged.	Cook-Inlet Gull (glaucescens x smithsonianus) 1st cycle, January 11 2007, Rio del Mar, CA (Jeff Poklen).

HYBRID GLAUCOUS-WINGED GULL X WESTERN GULL = "OLYMPIC GULL" averages bulkier and broader winged, with a more bulbous-tipped bill. Hybrids typically have more distinctly patterned greater coverts, less contrasting pale panel on inner primaries, and more uniformly dark tail.

Olympic Gull (occidentalis x glaucescens) 1st cycle, October 15 2006, Vancouver Island, BC (Guy Monty). Intense mottling with brown (not black) primaries.	Olympic Gull (occidentalis x glaucescens) 1st cycle, October 28 2011, Clallam County, WA (Steve Mlodinow). Note the dark mask of a Western Gull and the rather dark plumage overall with more GW Gull like primaries. Overall much bulkier than Herring Gull.

CONTINUE PART 4: FIELD IDENTIFICATION, SIMILAR SPECIES - SECOND CYCLE