|Note the white apicals on the female (left). The larger male (right) shows no white tips to the primaries.|
Here's the wingtip pattern to this apparent adult female that had settled for a "sub-adult" gull:
A thoughtful response I received from Maarten Van Kleinwee regarding this phenomenon of apparent sub-adults breeding:
"It is not exceptional of course for sub-adults (or perhaps better: those with sub-adult plumage) to take part in the breeding process. Last week I saw an adult male Black-headed Gull attempting to mate with a 2nd-calendar year Black-headed Gull. I have also seen 2nd-calendar year Black-headed Gulls on nests and with chicks. Examples exist also of sub-adult large gulls seen incubating".