I identified this Lesser Black-backed Gull Larus fuscus in the field (and later while processing my photos) as an immature bird, most likely a 3rd cycle / 4th-calendar year individual.
What led me to believe this were the black markings on the greater primary coverts, the alula and the tail, with the greater primary coverts in the underwing showing dark edges.
When I received the ringing details, I was therefore suprised to find out that this individual is already on its 4th cycle and in its 5th calendar year.
It is well known that black markings in the greater primary coverts of Lesser Black-backed Gulls cannot be safely used to separate immature Lesser Black-backed Gulls from adults (see Scoring primary feathers of Lesser Black-backed Gulls), but I would not expect to see the dark markings on the tail and underwing on a Lesser Black-backed Gull of this age as this individual is showing.
This once more shows the educational aspect of following ringed gulls and also stresses the point that we must be very careful in our eagerness to always want to accurately identify a gull down to the level of species and age when the exact details of that bird are not known. You just can never be sure what you’ve got in front of you.
All photos were taken on 1 April 2014 in Leiden, the Netherlands. The gull was fitted with a black ring, code ’52’, as a pullus in June 2010 in the Netherlands.
Also unusual to see (but not necessarily to do with immaturity): black markings all the way down to inner primary P2:
Typical 5th-calendar year
Here an example of a typical 4th-cycle, 5th-calendar year Lesser Black-backed Gull in the form of Yellow H.501, ringed in June 2007 as a pullus in Germany. The photo was taken 19 March 2011, Leiden, the Netherlands. Note the all white tail and the lack of markings on the greater primary coverts; basically indistinguishable from an adult.
Phenotypic characteristics and moult commencement in Dutch Herring & Lesser Black-Backed Gulls, Muusse, M., Muusse, T., Buijs, R.-J., Altenburg, R., Gibbins, C. and Luijendijk, B.-J., SEABIRD 24 (2011): 42–59.
Gull Research – Sub-adult Lesser Black-backed Gulls in October