OK. Fair question.
I lived on a boat for eight years. I had a pair of red-bills who 'adopted' me. Yes I did feed them.
The birds became very familiar with me and would alight onto the boat some 1/2 a nautical mile from my mooring and sit about a metre from me as I sailed onto the mooring. (I always said that pretend sailors have parrots. Real sailors have seagulls).
They were very obviously a pair, need I say more? The male had a thicker, more robust bill.
They eventually 'divorced' and the male showed up with a new partner, again with a slightly smaller bill.
How did I know it was a 'new' female? The old partner had lost part of her leg. She showed up at the boat several times when the newly formed couple were aboard and was always vigorously chased away by the male (the one with the more robust bill).
I have noticed this bill difference amongst other obviously paired birds and am sure that it is a characteristic example of sexual dimorphism exhibited by red-billed gulls.
I cannot comment on this secondary sexual characteristic re black-bills.