Scholars have not written as much about this tale and its history, but the widespread popularity of the tale lends credence to the theory that the tale was a popular tale before the Grimms recorded it. The first literary version of the tale was recorded by the Grimms and followed a few decades later by versions in Norway and Denmark.
The Grimms credited two sources for their version of The Six Swans published in 1812 in their original collection of tales. They credited Dortchen Wild as their oral source and the tale "Die sieben Schwane" from Feenmarchen (Braunschweig, 1801) as their literary source. The Grimms appeared to be fond of the tales which fall under the classification of AT- 451: The Maiden Who Seeks Her Brothers. While they often deleted extremely similar tales from subsequent publications of their collection, they kept three AT-451 tales, including The Six Swans, The Seven Ravens, and The Twelve Brothers. Jack Zipes theorizes that the tale was important to the brothers for its message about family fidelity through adversity and separation (Zipes 1988, 40).
Many variants of the tale abound including a Norwegian variant recorded by Asbjornsen and Moe, The Twelve Wild Ducks (1845), and The Wild Swans (1838), a version written by Hans Christian Andersen in Denmark.
When SurLaLune was first started in 1998, I considered adding The Six Swans fairy tale to the site. However, I could not find enough resources to merit its inclusion. Since then, a greater interest has been sparked in the tale by new novels, including Peg Kerr's The Wild Swans and especially Juliet Marillier's bestselling Daughter of the Forest. I recommend all of the fiction listed on the Modern Interpretations of the Six Swans page, especially Marillier's book.