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

(last update: December 2015)

Kim Seog-min (South Korea)
Hideo Shimura (Japan)
Peter Adriaens (Belgium)
Mars Muusse (Netherlands)

Slaty-backed Gull (schistisagus) sub-adult January

Identification and ageing

The immature birds on this page comprise two age classes: 3rd cycle (4CY January) birds and 4th cycle (5CY January) 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.


LENGTH 22-26.3 IN. (56-67 CM)


This large, dark-backed, pink-legged gull is a rare visitor to Alaska from Asia and occurs casually southward in N. America. Bill stout but relatively parallel edged, relatively slender on some females. At rest, tail tip falls about equal with, or slightly shorter than, tip of P7. Adult has dark slaty gray upperparts (Kodak 9.5-11.5); fairly extensive dusky brownish streaking on head and neck in basic plumage often concentrated around eyes. Juvenile medium brownish overall with mostly dark tail, relatively diffuse paler patterning, two-tone middle to outer primaries; plumage often bleaches strikingly over first winter. PA1 starts Oct.- Feb. Subsequent ages variable in appearance. All ages have pink legs; adult eyes pale lemon; orbital ring reddish.

No other dark-backed large gull occurs regularly in its northern range, so older immature and adult Slaty-backed Gull normally distinctive by virtue of dark slaty gray upperparts and pink legs, but see adult Vega Gull (medium gray upperparts contrast with black wingtips). Also see Western Gull (w. N. America) and Great Black-backed Gull (e. N. America). In first cycle also see Vega Gull, American Herring Gull, and note that various hybrids can resemble Slaty-backed Gull. See Similar Species and Rarer Species for details.


Usually considered monotypic, but subspecies ochotensis has been described from the Sea of Okhotsk, reportedly averaging darker above and slightly smaller overall than (nominate) schistisagus. (note 1). Has also been considered conspecific with Vega Gull because of interbreeding between the two in s. Koryakland. (note 2)


Ne. Asia, wintering South to Hong Kong, ranging to nw. Hawaiian Is. and w. Alaska.

Breeding. Has bred (1996-1997) in a Glaucous Gull colony at Cape Romanzol, w. Alaska (note 3) and may have nested on occasion in w. Aleutians. (note 4)

Nonbreeding. ln Alaska, generally rare in spring, summer, and fall on Bering and Chukchi coasts and on offshore islands in Bering Sea. Becomes locally uncommon (for example, on Seward Peninsula and St. Lawrence Is.) in fall (Aug.-Sept.). Casual to very rare (mid-May-Aug.) on Beaufort Sea coast east to Prudhoe Bay. In w. Aleutians, rare in spring (late Apr.-early June), very rare or casual in summer and fall. In cen. and e. Aleutians and s.-coastal Alaska, very rare or casual at any season; rare (mid-Aug.-mid-Mar., mainly late Oct.-early Dec.) in se. Alaska.

Elsewhere in N. America, casual to very rare (July-early Sept.) in Yukon and (mainly Nov.-Mar.) from B.C. south to cen. Calif. Scattered records (mainly Nov.-Mar.) of third-cycle and adult birds elsewhere in N. America, mainly east to Great Lakes region and s. Que. (note 5,6), and south to Colo., exceptionally to Tex. and s. Fla. Records from Atlantic Coast, and elsewhere in N. America, tainted by potential of hybridization with Vega and Glaucous-winged Gulls (see King and Carey (note 7).

BirdLife International

Distribution and population
The Slaty-backed Gull breeds in north-east Siberia (Russia) from Cape Navarin south to the northern tip of North Korea, including the Commander Islands, Russia and Hokkaido, Japan. Its winter distribution extends south to encompass Korea, the extreme north-east of China, much of Japan and Taiwan (del Hoyo et al. 1996).

Population justification
The global population is estimated to number c.25,000-1,000,000 individuals (Wetlands International 2006), while national population estimates include: c.50-10,000 wintering individuals in China; c.50-10,000 wintering individuals in Korea; c.10,000-100,000 breeding pairs and > c.1,000 wintering individuals in Japan and c.10,000-1 million breeding pairs and > c.1,000 individuals on migration in Russia (Brazil 2009).

Sakhalin coastline. This island in the northern Sea of Okhotsk hosts one of the largest colonies of Slaty-backed Gulls known in the Russian Far East. In 1986, the population was estimated at 2,000 pairs and increased to 5,500 pairs in 2005. Some colonies in the Russian Far East appear to be increasing dramatically. Verkhoturova Island in the Bering Sea, northeast of the Kamchatka Peninsula had 150 pairs in 1975, but by 1994, numbers had reached 4,800 individuals. The total world breeding population is estimated at 131,300 pairs (Larisa Zelenskaya, unpubl. data). The Russian population comprises the majority of birds with only 10,000 pairs nesting in Japan.


Slaty-backed Gull (schistisagus) 3rd cycle (4CY), January 02 2010, Choshi, Japan. Picture: Chris Gibbins.
Slaty-backed Gull (schistisagus) 3rd cycle (4CY), January 02 2010, Choshi, Japan. Picture: Chris Gibbins.
Slaty-backed Gull (schistisagus) 3rd cycle (4CY), January 02 2010, Choshi, Japan. Picture: Chris Gibbins.
Slaty-backed Gull (schistisagus) sub-adult, January 02 2010, Choshi, Japan. Picture: Chris Gibbins.
Slaty-backed Gull (schistisagus) sub-adult, January 25 2011, Choshi, Japan. Picture: Terry Townshend.