Reeper ..I've been thinking on this and well If you've already tried a diaphram call your half way there.....
1...slightly open your mouth and with out moving your lips or jaw...huff out air making ..like a high pitched HaHaHa sound
2...Now do that but... postion your tongue as if using a diaphram call...the slight movement of your tongue muscle..in the back of your throat.... will change the sound of the calling
3... once you can do that.... now you can learn to bring it together and start dropping your lower jaw to change things again...
This is for your basic yelps and clucks...I just can't explain how I get the other calls I mimic the wild birds making...it's just something that when I hear it...I can repeat it.....spit drums are pretty easy to do as well...If you try let me know how you do...Have Fun
