In the good old days before talking GPS machines, mariners often carried cages full of crows or ravens for use as aids in coastal navigation. If the sailors were uncertain of their position and bearing to a nearby land mass, they would release one of the birds who would usually fly in the shortest, most direct route toward the nearest land. In colloquial parlance, "as the crow flies" has come to mean the most direct route between two points, which usually is a straight line (as opposed to the route one might have to take to stay on roads or within established shipping lanes).