American Herring Gull (smithsonianus)

(last update: October 30, 2015)

Slaty-backed Gull (schistisagus) sub-adult, March 10 2012, Choshi, Japan. Picture: Peter Adriaens.

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Left bird is sub-adult Slaty-backed Gull (left, with slightly paler grey on upperparts) with Black-tailed Gull (centre, seems darker than the left bird in this image) and two adult Slaty-backed Gulls for comparison. Left bird not full adult because of smaller white primary tips and black mark on bill. More extensive winter streaking: head pattern with dark mask around eye and with distinct streaking on hindneck - is typical of Slaty-backed Gull, as is the bulky body. Wingtip black, unlike Glaucous-winged Gull. See also the flight shots of this bird, which reveals black in the tail. Primary pattern and dark underside of remiges typical of Slaty-backed Gull. Note that outerweb of P10 is blackish all the way up to the primary coverts, while the inner web has a distinct greyish bloom. Very distinct white tongue-tips on P5-9.

Relatively pale upperparts therefore seem to be the result of age and nothing else. Descriptions of mantle colour in literature (e.g. King & Carey 1999) refer mainly to full adult birds and do not take into account age-related variation.

Typical pattern on outer primaries, with blackish outerwebs of P8-10 (almost) reaching primary coverts. Many Slaty-backed Gulls show long, pointed white indentations on inner primaries, a result of the very wide white trailing edge there. Such white wedges are far less often shown by other large gulls (including Vega and Glaucous-winged Gulls).