A new feature for identifying adult American Herring Gull
by Peter Adriaens
Gulls never cease to amaze me. You can be studying one species for over ten years, and still find that you have overlooked something that was right in front of your eyes all this time. This is what happened to me when I came back from my second trip to Newfoundland, Canada, a few months ago and started studying my photographs.
Identification of American Herring Gull (in a European context) has been dealt with by Lonergan & Mullarney (2004) and Adriaens & Mactavish (2004). To all of the identification features described in those papers, a new one should be added, namely that of a grey ‘mirror’ on the underside of the outermost primaries. More precisely, this is a sort of isolated grey spot or grey hole inside the black(ish) pattern on the underside of P9 or P10 (see photos). It looks like a promising and easy feature for distinguishing between adult American and European Herring Gulls, though with some caveats.
The good news is that the grey ‘mirror’ is easily visible in the field (sometimes even on a standing bird), that it is seen in birds across the whole of North America (so not just in Newfoundland), and that it is rare in European Herring Gulls, perhaps mainly occurring in birds with medium to dark grey upperparts (i.e. slightly darker than American Herring Gull). The bad news is that it is shown by only a minority of the American birds, and that a few hybrids (or backcrosses) of Glaucous and Herring Gull in Iceland show it too, thus creating a serious pitfall for the unwary. A distinction should also be made between a grey ‘mirror’ – which is completely surrounded by the black colour of the feather – and a grey cut, which is not isolated (e.g. open at the feather edge; see plate 20 and 21). The latter pattern occurs regularly in both American and European Herring Gulls.
So, how many adult birds show this grey ‘mirror’ in North America? A quick analysis revealed the following proportions:
– Newfoundland: 58 out of 340 (= 17%)
– California: 14 out of 164 (= 9%)
Isolated grey ‘holes’ are therefore only shown by a small minority, and become probably scarcer towards the west of the continent, but when present they could serve as an indication for American Herring Gull and any bird with pale grey upperparts that shows this pattern may well be worth closer scrutiny. Especially those birds in which the isolated grey spot has a neatly rounded shape look distinctive. Nevertheless, it is always necessary to use as many characters as possible to clinch the identification.
In Europe (including the Azores), at least one claimed adult American Herring Gull has shown the pattern, namely the bird that has been wintering in Galicia, Spain, for at least six winters now. This bird shows an isolated grey ‘hole’ on the underside of P10 of the right wing (see here)
plate 1. Underwings of American Herring Gull (left) and argenteus Herring Gull (right). Note small, isolated grey spot on P9 in left bird. Photo left taken at St John’s, Newfoundland, in April 2000 (Bruce Mactavish); photo right taken at Heist, Belgium, in April 2011 (Peter Adriaens).
plate 2. Adult American Herring Gull, St John’s, Newfoundland, 26 Jan 2013 (Peter Adriaens). A bird with typical primary pattern, including a small, isolated grey spot on underside of P10, just in front of the white mirror.
plate 6. Adult American Herring Gull (with Kumlien’s Gulls, Black Ducks, and Greater Scaup), St John’s, Newfoundland, 24 Jan 2013 (Peter Adriaens). A rare example of a bird with two isolated grey ‘mirrors’ on P10.
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Full paper contains many more photos of American Herring Gulls, comparative European Herring Gulls and Glaucous X Herring Gull hybrids.
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Peter Adriaens, April 2013