The expression about killing two birds with one stone

Even in days "when killing birds was necessary to get food," I can't imagine that throwing stones at them was an very effective method. It wouldn't work: even a professional baseball pitcher can't consistently hit a spot the size of a bird. I'd think the only practical method would be trapping, as by limed twigs.

So if there was not a food-gathering practice of throwing stones at birds, where did the expression come from?