What is a "Yonat Elem Rechokim"?

Tehilla (Psalm) 56 begins, “For the conductor, on yonath elem rehokim,” and then goes on to describe David’s suffering during an episode where he was apparently captured by the Phillistines (this does not appear in the narrative text of Sefer Shmuel, the Book of Samuel.)  But what is Yonat Elem Rechokim?  Some speculate that it is a reference to the madness David feigned, where he was twitching and pecking like a dove (Yonah.)  It may also just be an instrument on which this Tehilla was played but which has now been forgotten.