To amplify what Jack says. A lot of songs have 'floating verses' so many songs about lovers end with a rose growing from one grave, a briar from the other and meeting in a lovers knot.
For the swan, how about Molly Bawn, where he shoots his sweetheart mistaking her for a swan.
A lot of folk stories have people turned into birds by an evil spell, and a faithful lover / sister rescuing them. Maybe that theme has got into song too.

For other weirdness, how about The Wind and the Rain

Plenty of ghost appearances in song, sailors coming to their lover in dream so that she knows he is dead, lovers telling the mourner to let him sleep after a year and a day. The Wife of Ushers Well.

And curses, Drowned Lovers

(I tried to make links to Mudcat threads and lyrics but it didn't work.)