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 February

The immature birds on this page comprise two age classes: 3rd cycle (4CY February) birds and 4th cycle (5CY February) 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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No other dark-backed large gull occurs regularly in the Bering Sea region, but beware Vega Gull, which can look deceptively dark-backed when seen against ice or snow. Vagrants need to be separated with care from a variety of potential confusion species and hybrids (see Rarer Species and accounts for Slaty-backed Gull hybrids).

Note in Slaty-backed Gull the slaty gray tone of upperparts, wingtip contrast and pattern, and overall structure; also see Slaty-backed Gull hybrids.

Slaty-backed Gull (schistisagus) adult, February 21 2014, Choshi, Chiba, Japan. Picture: Shiroto. Orbital ring pink, vivid red small gonys spot on pale yellow bill and yellow iris. Broad tertial crescents and secondary tips. Upperparts grey tone Kodak 8-11. Strong pink, short-looking legs.

Slaty-backed Gull (schistisagus) adult, December 25 2013, Shibetsu Harbor, Shibetsu, Hokkaido, Japan. Picture: Ian Davies. Small gonys spot, pale yellow base on bill and yellow iris with much speckling. Broad tertial crescents and secondary tips. Mirror on P10, but not on P9. Note string of pearls (white tongue tips) on outer primaries. Heavily marked head.

VEGA GULL (w. Alaska) averages lighter in build with more slender bill. Upperparts medium gray (Kodak 7-8) but can look darker against ice; note distinct contrast between upperparts and black wingtip, both perched and in flight, and more contrasting black wingtip on underwing; eyes olive to dirty pale lemon.

Vega Gull (vegae) adult, December 2009 - January 2010, Choshi, Japan (Chris Gibbins). Dark iris, grey upperparts are Kodak 7-8, which is paler than in Slaty-backed Gull.

Vega Gull vegae adult, December 05 2012, Choshi, Japan (Hiyashi Haka). Late moult, P7/P10, which indicates northern origin. Coral red orbital ring and gape. Very heavy winter streaking, down onto breast. Note large white neck patch above mantle.

Vega Gull (vegae) adult, January 21 2011, Japan (Stuart Price). Dark iris, heavily streaked head continuing on upper breast, grey upperparts Kodak 7-8, much black in wingtip (short tongue on P10 and full subterminal band on P10), pink legs. Vega Gull (vegae) adult, December 2009 - January 2010, Choshi, Japan (Chris Gibbins). Dark iris, late moult (P8 fully grown, P9-P10 growing), and pink legs. Note purplish orbital ring.

WESTERN GULL (w. N. America, regular in se. Alaska) has upperparts slightly paler or similar in shade (Kodak 8-11), but white primary tips smaller at rest, bill more bulbous-tipped and typically brighter in winter (orange-yellow, without dark distal marks), orbital ring yellow, eyes often dusky, and, in winter, has little or no dusky head and neck streaking. Also note Western’s narrower white trailing edge to secondaries (true?), narrower and more restricted whitish tongue-tips and mirrors on outer primaries, and blackish underside to wingtip.

Western Gull occidentalis adult, March 24 2007, Newport, Oregon. Picture: Greg Gillson. No winter streaking on head. Western Gull occidentalis adult, March 08 2009, off Dana Point, Orange Co. Picture: John Avise. Bill remains saturated yellow-orange, also in winter.
Western Gull occidentalis adult, November 19 2006, Crystal Cove, Orange Co. Picture: John Avise. Western Gull occidentalis adult, June 12 2011, Morro Bay, California, USA. Picture: Wikimedia Commons.

HYBRID GLAUCOUS-WINGED GULL X WESTERN GULL = "OLYMPIC GULL" (w. N. America) has paler upperparts (Kodak 6-8), typically a more bulbous-tipped bill, smaller white tongue-tips on outer primaries, and dusky (not brown-toned) and often smudgier basic head and neck markings.

Olympic Gull occidentalis x glaucescens adult hybrid, December 03 2008, Vancouver Island, BC. Picture: Guy Monty. Pigmentation on primaries intermediate, and grey tone upperparts intermediate.

Olympic Gull glaucescens x occildentalis adult, February 16 2009, Portland, Oregon. Picture: Lyn Topinka. Purple orbital ring and extensive speckling in iris ( >75%). Medium grey primary pigmentation, obviously darker than upperparts may be indication for occidentalis influences. Upperparts too dark for pure Glaucous-winged. Also primaries too dark, but not black as in occidentalis. Pure occidentalis would not show head mottling (at all in winter).


Slaty-backed Gull (schistisagus) 3rd cycle (4CY), February 20 2014, Choshi, Chiba, Japan. Picture: Shiroto.
Slaty-backed Gull (schistisagus) 3rd cycle (4CY), February 19 2014, Akkeshi, Hokkaidoi, Japan.
Slaty-backed Gull (schistisagus) 4th cycle (5CY), February 18 2014, Choshi, Chiba, Japan. Picture: Shiroto.
Slaty-backed Gull (schistisagus) 4th cycle (5CY), February 21 2014, Choshi, Chiba, Japan. Picture: Shiroto.
Slaty-backed Gull (schistisagus) (sub?) adult, February 10 2009, Chijiwa, Nagasaki, Japan. Picture: Shiroto.