An easier bird to age, taken at Niagara, Canada, 31 August 2010. The Mantle has the dirty brown markings of a first-winter bird, and in most respects this bird has 'classic' first winter plumage, the head streaking is lighter than you would get on a juvenile, and it doesn't have the scaly mantle and scapulars you would expect on a juvenile bird. Therefore, personally I would age this as a bird that has almost entirely completed it's moult from juvenile to first-winter plumage, which fits well with the time it was taken.
Identifying first-winter Ring-billed Gull in UK context
Identifying a first-year Ring-billed Gull in the UK is tricky, but do-able with good views. Despite typically having some brown in the mantle feathers, you can still see (as on the above bird) the paler grey mantle colour than a Common Gull. the grey crescents on the breast-sides and flanks are also a good indicator. In all plumages, Ring-billed is generally larger and bulkier than Common Gull, with a thicker, parallel-edged bill. It's head shape is typically squarer than Common Gull with a distinct peak behind the eye. They are also longer-winged, and have the jizz of a larger gull. Confusion can in fact be caused by second-winter Herring Gulls, but a direct comparison with larus argentatus should show a smaller, longer-winged bird, with a dark eye and more plaintive expression.
It is worth noting that the features of structure are often most obvious on a male first-winter Ring-billed Gull, many females can appear extremely similar to Common Gull, being around the same size, with a rounded head and more delicate expression compared to males. Given the number of adult Ring-billed found compared to the number of first/second winters, many, especially females, are doubtless overlooked for their similarity to Common Gull.
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