The swan maiden is a mythical creature who shapeshifts from human form to swan form. The key to the transformation is usually a swan skin, or a garment with swan feathers attached. In folktales of this type, the male character spies the maiden, typically by some body of water (usually bathing), then snatches away the feather garment (or some other article of clothing), which prevents her from flying away (or swimming away, or renders her helpless in some other manner), forcing her to become his wife.
There are parallels around the world, notably the Völundarkviða and Grimms' Fairy Tales KHM 193 "The Drummer". There are also many parallels involving creatures other than swans.
The folktales usually adhere to the following basic plot. A young, unmarried man steals a magic robe made of swan feathers from a swan maiden who comes to bathe in a body of water, so that she will not fly away, and marries her. Usually she bears his children. When the children are older they sing a song about where their father has hidden their mother's robe, or one asks why the mother always weeps, and finds the cloak for her, or they otherwise betray the secret. The swan maiden immediately gets her robe and disappears to where she came from. Although the children may grieve her, she does not take them with her.
If the husband is able to find her again, it is an arduous quest, and often the impossibility is clear enough so that he does not even try.
In many versions, although the man is unmarried (or, very rarely, a widower), he is aided by his mother, who hides the maiden's magical garment (or feather cloak). At some point later in the story, the mother is convinced or forced to give back the hidden clothing and, as soon as the swan maiden puts it on, she glides towards the skies – which prompts the quest.
Romanian folklorist Marcu Beza drew attention to two other introductory episodes: (1) seven white birds steal the golden apples from a tree in the king's garden (an episode similar to German The Golden Bird), or, alternatively, they come and trample the fields; (2) the hero receives a key and, against his master's wishes, opens a forbidden chamber, where the bird maidens are bathing.