Just for fun, here’s an overview of four 1st-winter Black-headed Gulls showing variation in their primary covert pattern.
The pattern ranges from the outer coverts fully white and inner coverts with just a dark shaft, to dark markings on the inner or outer shaft only, and finally to broad dark tips on all coverts.
All gulls were caught and ringed on the same day in December 2017 in Leiden, the Netherlands and are of unknown origin.
Many thanks to Frank Majoor for allowing me to take these photos during his ringing session.
Below is each individual in full.