E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Swan, a public-house sign, like the peacock and pheasant, was an emblem of the parade of chivalry. Every knight chose one of these birds, which was associated in his oath with God, the Virgin, or his lady-love. Hence their use as public-house signs.
The White Swan, a public-house sign, is in compliment to Anne of Cleves, descended from the Knight of the Swan.
Swan with Two Necks. A corruption of Swan with Two Nicks. The Vintners Company mark their swans with two nicks in the beak.
N.B. Royal swans are marked with five nickstwo lengthwise, and three across the bill.