Thanks for all the replies. I sure learned a lot this year about how sensitive later-season roosters are to noise. In past seasons (this was only my third) I hunted alone, and was not experienced enough to understand that Ruby was on scent of birds that had high-tailed it outta there with all my whistling and Ruby's bell clanging away (she wears one during deer season). This year I started hunting with a buddy, and we sometimes did some blocking in the late season. It was amazing how he would shut his car door at the far end of the field, and a minute later, almost before he started into the cover, the birds were running out of the far end!
I now use an orange vest on Ruby where deer hunters are a concern, instead of the bell, and we have been working hard on much quieter control/commands. I bought an ultrasonic whistle, which she responds well to, so that was why I was asking if the pheasants would hear it too or if it was just silent to humans! I'll keep digging around for any info on the specific frequencies that pheasants can detect - I'll post an update to this thread if I come up with anything new.
-Croc