Spectator sentence of the year

From Dot Wordsworth’s language column about Bishop Heber’s hymn “From Greenland’s Icy Mountains”:

He was a clever man and agreeable, leaving an account of the once-a-century Mallard ritual enacted on the rooftops of All Souls College, Oxford, in 1800, but dying upon taking a bath as Bishop of Calcutta, aged 42.

Are you as curious about what the “Mallard ritual” might be as I am?

UPDATE:

Thanks to Stephanie in the comments we learn that the Mallard ritual involves the Mallard Song, which “was sung after a rude manner about 1658 about 2 or 3 in ye morning, which giving a great alarm to ye Oliverian soldiery then in Oxon they would have forced ye gate open to have appeased ye noise”. 

The lyrics may be found here at the Mallard Society’s website, which warns that the fifth verse “was expunged on grounds of decency in 1821”. If you are bold enough to explore, you will realise that “swapping” in the lyric meant something different in Middle English from what it means now.