1st-year bird, Kenniskis, Canada, 15 August 2010. For a first-winter bird, this individual is relatively unmarked. Given the time of year, it seems likely to be in moult, which would explain the lack of markings. Olsen and Larsson state that moult from juvenile to first-winter plumage takes place from July-September, and from first to second-winter plumage from May (in some extreme cases March)- October.On average, a 1-2nd winter moulting bird would have completed all but it's primary moult by mid-August when this was taken. Without seeing the wings very well in this photo this can be quite difficult to determine, but from what can be seen of the lesser and median coverts they appear to be in first-winter, not second-winter plumage, which would support the idea of a bird undertaking it's first moult. the crescent marks on the sides of the breast are a good sign of a first-winter bird and aren't normally shown on a first-summer/second-winter. Therefore I believe this bird to be a juvenile moulting into first-winter.
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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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