“As the crow flies” refers to the most direct route from one place to another. The idiom describes a route that is in a straight line from point A to point B. It is not a route that must stick to a road, go around obstacles, or stop at a body of water.
Most language historians attribute the phrase to literally coming from the route a bird could take when it flew from one location to another. As the crow flies, neither natural barriers such as mountains nor political barriers such as borders would be an issue in the journey from one location to another. There is an alternate theory as to the origin of the saying, however.
Some historians cite the phrase coming into common language from life aboard British ships that worked along the coasts. According to this theory of origin, sailors kept a cage of crows aboard the ship. In a deep fog, when the sailors couldn’t be certain of where the nearest land might be, one of the crows would be released from the cage. The bird would immediately fly in a direct line for land, and the sailors could follow it to get to safety.
The first use of the idiom to be found in writing appears in 1767. W. Kenrick uses the phrase in The London Review of English and Foreign Literature. The phrase is used in the text to describe a direct route that isn’t impeded by water or mountains.
An idiom from Scotland is similar to and came into use about the same time as the English phrase. The Scottish describe the most direct route from point A to point B as a crow road. The first written reference to a crow road is believed to be 1795.
As with many idioms, "as the crow flies” has found its way into literature and musical references. Jeffrey Archer’s 1991 novel was titled As the Crow Flies. A 1986 play by David Henry Hwang is similarly titled.
The crow figures into several phrases in the English language. In addition to “as the crow flies,” these include “a crow to pick” and “eat crow.” Most likely the crow is so often used as reference in these sayings because the crow is a large, loud, and very noticeable bird with which most people would be familiar.