whistle
Whistle just came to me after a while, though even after a few months (though I have not been practicing it that much) I am still not in complete control - as in I break between high headvoice and whistle - it takes time and experimentation. Don't worry that you don't have it as some materials suggest, everyone has it since it's just another coordination, it just takes time to discover and use well.
I suggest Adam Lopez for impressive use of whistle voice...
highest vocal note ever by a male:
media.putfile.com/Guinnes...-by-a-male
his website:
www.adamlopez.com.au/