Niles was, to put it mildly, a very creative modifier of existing folk material, and an inveterate inventor of new. Most of our serious scholars wisely discount his claims to have "collected" much of his material. I'm accustomed to taking everything he wrote with a very large grain of salt. He did write some very beautiful songs, regardless of his many prevarications.

There is a version of the Corpus Christi Carol in the Oxford Book of Ballads entitled "The Falcon" after its final refrain line: "The falcon hath borne my make away." MacEdward Leach prints that text, along with another of the "Down in yon forest" variety in his Ballad Book. Either of these books ought to be easily available at your library or through inter-library loan. Bruce O. will probably come up with half a dozen more specific references, but these should get you started. Gustav Holst, by the way, wrote a lovely setting for the Falcon text.

Sandy