Monday, 22 April 2013

Introducing Isabel - a dilution mutation Herring Gull at Seafield

Herring Gull (centre) with dilution mutation (isabel) showing much reduced eumelanin across plumage
Seeing a white-winged gull is usually the highlight of any day's birding for me. Sunday was no exception and this time the bird was a Herring Gull, albeit one with a dilution mutation (as defined by van Grouw in the January edition of British Birds). This mutation results in a quantitative reduction of melanin - and since gulls only have one form of melanin (eumelanin) it is impossible to say whether this mutation is "pastel" (the reduction in both forms of melanin) or "isabel" (the reduction of eumelanin only).

At rest most of the plumage appeared to be snow white. Bare part colouration appeared to be normal.
This gull - let's call her Isabel, perhaps - has been reported at Seafield already this winter, but this is the first time I have seen her. Why her? Well, many of these conditions are sex-linked in birds, and since in birds the heterogametic sex is females rather than males (unlike mammals, where males only have one copy of the X chromosome), so I am guessing that it might be more likely that this bird is a female.
Massive crop, but gives a good idea of the bird in the field
I'm looking forward to closer views of Isabel at some point in the future... Here are some more flight shots - the normal grey upperparts are white and the black wingtip a nice faded grey.
The dark smudge on the underwing in this shot is a shadow of another gull.
Again, a shadow is unhelpful but the white plumage with grey on P8-10

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