In the introduction the author invokes a quote from Juvenal's satire Rara avis in terris nigroque simillima cygno ! " This is an ironic quote that translates
" A rare bird in the lands and very like a black Swan".
What he meant by this quote was something whose rarity would compare to that of a black swan or in other words as a black swan did not exist in Europe , neither did the supposed characteristics of the rare bird with which it was being compared.
THE BLACK SWAN IS A METAPHOR FOR SOMETHING THAT DOES NOT EXIST.
The author of Willobie his Avisa specifically states that chastity in women is as rare as a black swan 9In other words it does not exist).
The author also refers to Avisa as being" a not seen bird ."
The author is ironically implying that Avisa is a" not seen bird" or a black swan because chastity DOES NOT EXIST in AVISA. The concept of the "not seen bird " is of critical importance in understanding the book.