Sunday, 4 January 2015

Herring Gull with a grey 'pseudo-mirror' on the underside of P9

A grey mirror found on the underside of P9 or P10 has been suggested as a supporting micro feature for distinguishing adult American Herring Gulls from European Herring Gulls. It is described as a somewhat isolated grey spot or grey hole inside the black pattern on the underside of P9 or P10. The grey spot is either completely surrounded by black or seen as a grey cut which is open towards the base of the feather. It may be present in both wings at the same time or only in one of the wings.

The feature is apparently rare in European Herring Gull, but has been seen in a few hybrids (or backcrosses) of Glaucous Gull x Herring Gull in Iceland. During Christmas holiday this year I looked through some more of my photos of apparent argentatus Herring Gulls, and came across the individual below which was seen on December 28, 2008 in Østerby Harbour on the island Læsø in Denmark. It represents the second European Herring Gull that I have ever documented with such a small grey ‘pseudo-mirror’. The first example was this one from the other end of the same island.

Another interesting character in this individual is that there is no mirror on P9. However, it is widely known that there is extensive variation in the primary pattern of European Herring Gull. We also know that precise pattern changes over years as documented by Maarten van Kleinwee

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