I’ve heard a couple of times that this expression came from when sailors felt they were close to land they would release a crow. If the crow came back, there was no land nearby but if the crow did not come back, they would travel in the direction the crow flew. There are other variations of what the expression meant. As many already know, the expression “As The Crow Flies” simply means the shortest distance between two points which would be a direct line. When crows (or most other birds for that matter) fly they do not have obstacles we would have on the ground such as mountains, water bodies and such. When you set your GPS to “Off Road” it measures the distance as the crow flies.
The earliest known citation of the phrase, which explicitly defines its meaning, comes in The London Review Of English And Foreign Liturature, by W. Kenrick – 1767.
So now you know! Enjoy the cache.