Thayer's Gull (thayeri)
(last update: January 22, 2013)
Slaty-backed Gull schistisagus 1st cycle (1CY), December 25 2009, Choshi, Japan. Picture: Chris Gibbins.
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Almost all scaps replaced. GC uniform brown with limited patterning. Tail largely dark.
Some similarity to Thayer's Gull, but SLATY-BACKED GULL (n. Pacific) should be separable by structure, being larger and bulkier with broad wings, shorter wing projection, longer legs, and stouter bill. Its plumage, however, is similar to Thayer’s, although the upperparts tend to be more uniform and less contrastingly patterned (suggesting a dark Glaucous-winged Gull), tail averages darker, and dark/light contrast on outer primaries averages less distinct at shaft.