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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Small flock of Slaty-backed and Vega Gulls, showing variation in mantle colour. Of the four Slaty-backed Gulls in the centre, the three birds to the right (especially the one at the back) are discernibly paler than the adult bird. They show blackish mark on bill and/or small white primary tips, and are therefore subadult types. Note thick, 'inflated' body compared to Vega Gulls in the same flock.

Subadult age of two centre birds confirmed by brown tinge on tertials. Three birds right obvious paler grey on upperparts. The breeding range of Slaty-backed Gull does not really overlap with that of Glaucous-winged Gull and hardly with that of Vega Gull (see http://publications.gc.ca/collections/Collection/CW69-15-4-2000E.pdf#page=38 for details). Only very low numbers of mixed pairs have been reported (e.g. only one Slaty-backed x Glaucous-winged Gull pair in a colony of 3,000 pure pairs on Toporkov Island in 1993 - see King & Carey 1999). It therefore makes little sense that a small wintering flock like the one in this photograph would contain a majority of hybrids, standing shoulder to shoulder. The variation in mantle colour is more likely age-related.