[Folklore] According to the mythology of pre-Roman Britain, if you can get the feather of the Swan May, you can perform great feats of magic, including turning yourself into a swan. But beware the Swan May, if she should get her feather back, she can escape.
SWAN MAY
Meaning of SWAN MAY in English
Encyclopedia Mythica English dictionary. Английский энциклопедический словарь греческой мифологии. 2012