Swearing by Swans

"At the banquet held on this occasion, he vowed before God and the *swans*, which according to usage were placed on the table, to punish the Scottish rebels."—Keightley's *History of England*, vol. i. p. 249. ed. 1839. What authority is there for this statement respecting the swans? What was the origin and significance of the usage to which allusion is here made?