Images of European Herring Gull Green YCKL in its fifth calendar year, but showing a fourth-calendar year plumage: much brown on the wing coverts, primaries, and tail, and much black on primary P5.
This reminded me of 2020 when it showed the plumage of a third-calendar year when it was in its fourth calendar year: much brown in the primaries and secondaries, a distinct tail band, and no white in the primaries.
Its plumage therefore has been a year behind what is expected to be seen in a Herring Gull of that age.
YCKL was ringed by us as a pullus in 2017 in the IJmuiden Forteiland colony, the Netherlands, and we therefore know its exact age.
I have not come across many birds that show such a retarded plumage, but it does mean that ageing sub-adult Herring Gulls can be tricky when they are not ringed and their exact age is not known.
I took the following images on 12 April 2021: