Having had a closer look on a full size screen (as opposed to my phone) the black mark actually appears to be on the tertials or secondaries but in any case indicates immaturity.
John first thought it was a Yellow-legged Gull in the field and lets face it, our first instincts when calling a bird are usually the right one. It's only when a bird gets over scrutinised that mistakes can creep in. The overall jizz, bill shape and upperpart tone look OK for YLG. Some YLGs can show a small dark eye like the Upton bird.
Without seeing the primary pattern properly, it would be a brave person to say definitively that it was YLG or Caspian or even a hybrid but my hunch is for YLG. Out of interest a presumed hybrid Caspian/Herring was seen in Staffs last month which had a small dark eye - pic
here. The whole 'white-headed' large gull complex is a minefield!
I saw a possible candidate for an adult-type Caspian Gull at Throckmorton briefly on Monday while looking for the Glauc but this was not the same bird that was seen there yesterday and neither did it look anything like the Upton bird.
Brian
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