Mine do the whistling, too! I swear I thought I heard my hens doing it too, though! Could have just been me.
I'm going to try to get a video of this. My husband thought the females made the sound as well, but I have never heard my females do this one particular call. The females will make other calls along with the "buckwheat" or "come back", and sometimes they both do a quivering type of whistle that almost sounds like they are distressed, but my males are the only ones making this "sur-real" whistling call.
Patti - that's interesting that Spider is more vocal. Everyone always talks about the females being vocal and your Mary Jane is the quiet shy one.
Starting to feel like the Jane Goodall of the Guineas!