Who does the backing vocals on this song?

Even my super spectacular special edition of 'Sweetheart...' doesn't even bother to mention it.

It sounds like a lady, and I wondered if it might be old Emmylou. But I heard he didn't meet her til later on.

Is it one of the other Byrds, even? It always gets me wondering whenever I hear it on my iPod - which is very often, so I'd be grateful if someone could clear it up.