I sucked at spanish in HS & college because I can't roll my "R's" to save my life, but growling over a reed, THAT I could figure out. A deep throat growl as you blow will throw a lot of "trill" into the call. Over lower pitched reeds, it'll throw rasp. Over a split reed like the Syco Tweety, it'll shred the sound apart like nothing you've ever heard.
If I had to think of a word I was "saying" during a bird distress call, maybe "greeeet" or "greht". The "GR" at the front gets the reed shaking, then the eh or ee sound lets the reed roll the pitch, and the "T" at the end puts the sharp stop that bird sounds tend to have on the back end.