Let me give it a whirl...
When lost or unsure of their position in coastal waters, ships would release a caged crow. The crow would fly straight towards the nearest land thus giving the vessel some sort of a navigational fix. The tallest lookout platform on a ship came to be know as the crow's nest.
Thanks for the trivia, Andrea.
it came for air travel,air mileage is "shorter" because it is srait,like how a crow flies. land travel is "longer", up and down hills and arround bends. A mile is a mile but it takes less miles thru the air because its like how a crow files..strait?