See Swan Upping (above) ...

"The Dyers and the Vintners Companies are now the only owners of private swans on the Thames, the Worshipful Company of Dyers marking theirs with a nick on one side of the beak, and the Worshipful Company of Vintners marking theirs with a nick on each side. The latter is the origin of the inn sign 'A Swan with Two Necks'. (i.e. two 'nicks'). Royal swans are now left unmarked."