This first-cycle, second-calendar year Common Gull Larus canus shows distinct white markings on P10, reminiscent of the mirrors that are acquired in the second cycle.
2 March 2014, Benthuizerplas, Zoetermeer, The Netherlands.
2 March 2014, Benthuizerplas, Zoetermeer, The Netherlands.
2 March 2014, Benthuizerplas, Zoetermeer, The Netherlands.
The only time that I have come across something similar was in the following individual, although here the marks were just tiny spots, including one on P8:
13 July 2013, Benthuizerplas, Zoetermeer, The Netherlands
Gulls to the Horizon 