Together, the Stretchback Grunt Call and Micro Heat Bleat Call, by Duel Gamecalls, allow you to make every deer vocalization. This package includes: The Duel Stretchback Grunt Call and the Duel Micro Heat Bleat Call. Duel's patented Dual Chamber Technology projects sound through two separate chambers. This emulates the oral and nasal cavities through which sound is projected by real animals. The result is a tonally robust, resonating sound that's better at convincing game you are real.