The most reliable call I know of goes in your mouth. It's that 6" BMT on wheat bread from Subway that you packed in your cargo pocket.
This is practically guaranteed to work almost every time. See, it's like 10:30 and your belly is rumbling because all you've put in it was that cup of coffee at 5:00 AM. You'd of ate something earlier, but you slept a little longer than you intended and you were running late. Ooops.
So there you are, sitting against your tree. All the action has drifted off somewhere else and you're certain there's nothing going on in your corner of the woods. ....You survey your surroundings one last time and you lay that shotgun in your lap. You retrieve that sammich from your pocket and take a big bite. ....Uhmmm, so good. .....One more big bite and you get the feeling you're being watched.
Then, over there on the left at about 70 yards. There's a big red head looking at you like, "Hey, what-chu got on that sammich ?"
Give it try, this spring.