Whatever they sense it is a trigger for them to mean feeding time. I hunted and area next to a hunting lodge that sold high dollar trips. They had teenage boys driving around to the green fields with truck beds full of corn. Scoop a couple of buckets of corn out on the ground. Beat the bottom of the bucket like a drum and drive to the next one. Before they were out of the field the deer, hogs, and turkey would be headed that way. It was an amazing and sad sight to behold.