I love box calls. Not sure they're used much anymore. I have 8 or 10. IMHO, nothing produces a more realistic yelp than a good box call.
Truth.
I'm overwhelmingly a mouth call guy. But I'm never, EVER without a box call in the woods. Got to the woods some day with a guy who's good with mouth, box and pot & peg. Go out 50-75 yards and have him run all 3. You'll come home with a new-found respect for a box call.
The other thing about a box........and it's already been mentioned.......is the distance/volume factor. In big country where you fins Merriam's, Rios and Gould's the volume of a box is a huge advantage. Likewise, even for Easterns in rolling terrain or on a windy day.............box call gets to their ears better than any other call.
