Interesting to see the development of the bill colouration in this fourth-calendar year European Herring Gull during various months of the year.
This gull was caught as a breeding male in the gull colony of IJmuiden Forteiland, the Netherlands in May 2016 and ringed as Green Y.CAX.
In May, the bill showed sub-adult features with much black in the lower and upper mandible, a dark yellow base and a pale tip, typical for a Herring Gull of this age.
In August (so after the breeding season), the bill looked very mature with only a small remnant of black on the bill.
In October (and photographed in November), the bill has turned very immature with a pale pink base, a distinct black band across the bill and a yellow tip. Personally I was not expecting to see such a dramatic change.
Below follow the images in larger size, the transformation to non-breeding plumage is striking. Many thanks to Mars Muusse for providing the photo taken in November.