It's often used, when giving distances, to mean "literally between one point and the other, not counting how far it is when you take the geography into account". That's quite a complicated definition, but it's quite an easy definition. I live seven miles *as the crow flies* north of a big city. But since I have to drive the routes dictated by the roads, I may drive ten miles before I actually get into the city. "How far is it to London from here?" "As the crow flies, about four hundred miles".

Australians who live in the outback often use this expression as there aren't any signs for hundreds of kilometres. It means by the shortest and most direct route. Example " Mate, the next town is 300 kilometres as the crow flies ."

A crow is a black bird and the expression refers to it flying in a direct line from A to B without the detours we humans have to take,when following a road.It is an odd choice though ,because crows in their migration don't necessarily fly straight and many times fly in wheeling arcs in their quest for food. So the expression indicates the most direct route that could be taken from A to B. When using it ,it indicates an approximate estimation of the distance way lesser than the real distance crossed,due to encumbrances of roads and landscape features that man is hampered by. The earliest known citation of the phrase, which explicitly defines its meaning, comes in The London Review Of English And Foreign Liturature: " The Spaniaad , if on foot, always travels as the crow flies, which the openness and dryness of the country permits; neither rivers nor the steepest mountains stop his course, he swims over the one and scales the other."