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) 3rd cycle / 4CY May

3rd generation flight feathers. 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".

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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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Also see Great Black-backed Gull, Kelp Gull, Lesser Black-backed Gull, and (as immatures) Yellow-legged Gull and European Herring Gull.

Adult Cycle.
Note slaty gray tone of upperparts, leg color, wingtip pattern, and overall structure, especially bill size and shape.

YELLOW-FOOTED GULL (Gulf of Calif.) has slaty upperparts similar to Slaty-backed but stouter bill more swollen tipped, legs yellow, wingtips more extensively blackish with no bold pattern of white tongue-tips. Prebasic molt averages 2-3 months earlier than Slaty-backed.

Yellow-footed Gull (livens) adult, January 30 2009, La Paz, Baja California, Mexico (Amador). Already on structure not like Herring Gull. Yellow-footed Gull (livens) 1st cycle, August 09 2010, Salton Sea, CA (Amar Ayyash). Dark inner primaries.

First Cycle.
Note overall size and structure, bill size and shape, two-tone pattern of outer primaries, relatively plain greater coverts.

YELLOW-FOOTED GULL has more swollen-tipped bill, darker upperwings lack pale panel on inner primaries; PA1 molt often extensive, including upperwing coverts.

Second Cycle.
Note overall size and structure, bill size and shape, two-tone pattern of outer primaries, incoming slaty gray feathers on upperparts.

YELLOW-FOOTED GULL has flesh to yellowish legs, upperparts mostly slate gray (like third-cycle Slaty-backed) but tail mostly black or mixed black-and-white, bill flesh to yellow with black distal third in winter. Prebasic molt averages 2-3 months earlier than Slaty-backed.

Third Cycle.
Differs from other species in much the same respects as adult cycle (which see); note wingtip pattern, bill size and shape, tone of upperparts, leg color.


Much as other large gulls such as Glaucous, Vega, Glaucous-winged, and American Herring, with which it usually occurs at feeding and loafing areas.


Dark areas of plumage in this species, such as upperwing coverts of first-cycle birds and basic head streaking of adults, seem especially prone to bleaching. However, the wide variation in adult upperpart tone attributed to this species by some authors (note 8) is presumably due to hybridization with other taxa. (note 9)

Adult Cycle.
Complete PB molt (June/July-Dec./Jan.) produces adult basic plumage: head, neck, and underparts white; head, neck, and sometimes upper chest with variable dusky brown to warm brown streaking and mottling often concentrated around eyes. Upperparts dark slaty gray (Kodak 9.5-11.5) with black wingtips (black on outer webs of P9-P10), sometimes also P8, extends to primary coverts), broad white scapular and tertial crescents, and broad white tips to outer primaries. Uppertail coverts and tail white. Broad white trailing edge to secondaries (often shows as a skirt) and inner primaries breaks into discrete white tips on outer primaries; P5-P7/P8 have white tongue-tips; P10 or P9-P10 have white mirrors. Underwings show dark slaty to silvery black wingtips (with white primary tips, mirrors, and tongue-tips) blending into white-tipped, dusky gray inner primaries and secondaries, which form a dusky subterminal band on underwings. Eyes pale lemon (rarely dark), orbital ring reddish. Bill yellowish or variable fleshy at base with orange-red to red gonydeal spot and sometimes a dark distal mark. Legs rich pink, perhaps paler during height of PB molt. Partial PA molt (Oct.-Mar./Apr.) produces adult alternate plumage: head and neck clean white (rarely before Mar./Apr.). By spring, orbital ring, bill, and legs brighter, bill lacks dark distal marks; white primary tips reduced through wear.

First Cycle.
Juvenile (Aug.-Feb.): medium brown to medium dark brown overall. Head, neck, and underparts variably streaked and mottled whitish, becoming white on vent and undertail coverts, which have brown barring. Upperparts with buff (bleaching to whitish) scaly and notched edgings, the greater coverts typically fairly plain or with variable, often fairly coarse but diffuse, distal whitish notching. Tertials dark brown with notched whitish tips and distal edging, the dark tertial bases merging with secondary bases that often show as a skirt. Uppertail coverts whitish with brown barring. Wingtips blackish (bleaching to dark brown) with narrow whitish fringes to tips; contrastingly darker than tertials. Outer primaries blackish brown with paler inner webs often forming a two-tone pattern on backlit spread wing, merging inward with pale inner primary panel. Secondaries blackish brown, tipped whitish. Tail blackish brown overall with narrow whitish tip; variable, wavy whitish bars and marbling at basal corners; often irregular white ribbing on outer web of R6; can show a broad, dark distal band from below.

IN FLIGHT; brownish overall with mostly dark tail, dark secondary bar (often contrasting strongly with pale greater coverts in faded plumage), and pale inner to middle primary panel blending into outer primaries, which, when bleached, can show distinct two-tone pattern;  underwings medium brown with remiges often reflectively pale. Eyes dark, bill blackish and often with some dull pinkish basally by midwinter, legs dusky flesh to flesh pink. PA1 molt (Oct.-Apr./May; does not include upperwing coverts) produces variable first alternate plumage: head, neck, and underparts often bleach to mostly whitish, with variable brownish (worn juvenal) to smoky gray (A1) streaking and mottling. A1 scapulars typically brownish to grayish (often fading to whitish) with dark centers or shaft streaks; less often with anchor patterns or subterminal bars. Blackish brown to dark brown wingtips contrast with faded upperwing coverts; greater coverts often bleach to a plain, whitish panel. Bill typically shows some dull-flesh to pinkish basally, sometimes a pale horn tip; legs flesh pink. Note: Some birds retain most or all juvenal plumage through late winter and can be very bleached, almost whitish in overall appearance except for flight feathers.

Second Cycle.
Complete PB2 molt (May/June-Oct./Nov) into second basic plumage. Head, neck, and underparts streaked and mottled whitish and dusky brown, often whiter overall on head and chest; undertail coverts with sparse dark bars. Upperparts medium dark gray-brown overall with diffuse whitish to pale brown edgings and tips to wing coverts and scapulars, often a few dark slaty scapulars and mantle feathers; tertials blackish brown to dark brown with broad whitish to marbled pale brownish tips. Wingtips blackish with fine pale fringes, most distinct out to P5/P6. White uppertail coverts have sparse dark marks and contrast with narrowly white-tipped, mostly blackish tail; outer rectrices with white basal corners more diffuse and speckled than first cycle. Upperwing pattern of remiges similar to first cycle, but outer primaries have more distinct pale tongues, inner primaries paler overall, secondaries average paler and do not always form a contrasting dark bar. Wing-linings medium brown with underside of remiges often reflectively pale. Eyes brown to pale grayish, bill averages more extensively pinkish at base than first cycle and often has small pale horn tip; sometimes extensively flesh pink on basal two-thirds with blackish tip or subterminal band. Legs flesh pink. Partial PA2 molt (Sept.-Apr.; can include some upperwing coverts) produces second alternate plumage: head, neck, and underparts whitish with variable dusky mottling and streaking, often concentrated as a dusky belly smudge; head, neck, and chest often bleach to mostly white. Back becomes mottled to almost solidly slaty gray in contrast to faded whitish and brownish upperwing coverts; some birds attain a few slaty gray upperwing coverts, especially median coverts. Eyes pale lemon to brown, orbital ring can be reddish. Through winter, bill typically pinkish basally, blackish distally; in summer some have a yellow bill with reddish gonydeal spot and black distal band. Legs flesh pink.

Third Cycle.
Complete PB3 molt (May/June-Nov) produces third basic plumage: resembles adult basic overall but with more extensive dusky streaking and mottling on head, neck, and underparts; upperwing coverts mixed slaty gray and brownish; white scapular and tertial crescents and tips to remiges narrower (but still broad); P10 mirror smaller and P9 mirror absent; smaller and more diffuse white tongue-tips on P5-P7/P8; white tail typically has fairly broad, often broken, blackish distal band but can be all white or with small dark subterminal marks; white wing-linings lightly washed and mottled with brownish. Bill ranges from adultlike with a blackish subterminal mark to pinkish with a black subterminal band. Partial PA3 molt (Sept.-Mar./Apr.) into third alternate plumage: dark markings on head and neck reduced to absent: bill brightens by summer and often indistinguishable from adult, or with dark distal marks. White tips to outer primaries can be lost by wear. Adult plumage attained by complete PB4 molt (June-Nov./Dec.) although some fourth-cycle birds probably show dusky marks on tail and less white in wingtips.


With Vega Gull in Siberia, (note 10) with Glaucous-winged Gull in Kamchatka and on Commander Is. (note 11,12,13)

1. Vaurio 1965:735; 2. Ibid.; 3. McGaffery et a1. 1997; 4. Gibson and Byrd unpub1. data; 5. Goetz et a1. 1986; 6. Gustafson and Peterjohn 1994; 7. King and Carey 1999; 8. Gustafson and Peterjohn 1994; 9. King and Carey 1999; 10. Vaurio 1965:735; 11. Firsova and Levada 1982; 12. Grabovsky et a1. unpubl. data; 13. King and Carey 1999.

Slaty-backed Gull (schistisagus) 3rd cycle (4CY), May 02 2010, Ayukawa - Kinkazan, Japan. Picture: Shigenobu Aizawa.
Slaty-backed Gull (schistisagus) 3rd cycle (4CY), May 14 2014, Choshi, Chiba, Japan. Picture: Shiroto.