While we wait for Ghostisfort to tell us more about logic, etc, I'll post this, which just dropped into my inbox in a World Wide Words email, and which coincidently links to my last post:
Q and A: As the crow flies
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Q. I've come across an interesting suggestion for the origins of
the expression "as the crow flies". It's said it has its roots in
something called "raven flocking", a method medieval sailors used
to find land. They supposedly kept a raven or a crow on board ship
and when the sailors thought they might be near land, they would
let the raven or crow loose and would assume land was in the
direction that the bird flew. Is this true? [Lynne Spear]
A. It's amazing how people can make a simple topic complicated in
the search for a good story.
I've not come across "raven flocking" and can't find a reference to
it anywhere. So far as I know, adult ravens don't flock: they mate
for life and defend a territory. Crows don't flock either, though
the closely similar European rooks do, being gregarious birds that
nest in colonies. (As a bit of British ornithological trivia, an
old adage has it that you can always tell a crow from a rook, even
at a distance: if there's one bird, it's a crow, if more than one,
they're rooks.)
You sent me a link that your husband found to a website of sailing
trivia. It explains the expression in a related way:
The term "As The Crow Flies" came from British coastal
vessels that customarily carried a cage of crows. Crows
detest large expanses of water and head, as straight as a
crow flies, towards the nearest land if released at sea -
very useful if you were unsure of the nearest land when
sailing in foggy waters before the days of radar. The
lookout perch on sailing vessels thus became known as the
crow's nest.
I'd hate to see a cage of crows: the birds would probably peck each
other to death. And the birds must have had supernatural powers, to
be able unerringly to see land through fog. You can tell this is
folk etymology through its linking of the story to the crow's nest,
which has no etymological connection with "as the crow flies". The
crow's nest was given that name because, like the nest of a crow in
a tree, it was perched high on the mast.
The expression can't be from medieval times, because it's recorded
only from the eighteenth century. And all early instances refer to
directions on land with no mention of the sea.
The true explanation lies in a bit of British country lore that's
based on observation of the birds. Anyone who has watched a crow
flying any distance knows it tends to do so in a steady, unwavering
line - not always, but then this is a generalisation of a tendency,
not invariable fact. Since the flight of the crow is unaffected by
obstacles on the ground, its route came to represent the shortest
distance between two points.
This is the earliest example I've so far found:
Now the country that those Indians inhabit is upwards
of 400 miles broad, and above 600 long, each as the crow
flies.
[The Gentleman's and London Magazine, Dec. 1761.]
And this slightly later one makes the link explicit:
The Spaniard, 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,
and by this means shortens his journey so considerably,
that he can carry an express with greater expedition than
any horseman.
[The Political Magazine, Nov. 1782.]
I also found this, with more about Odin and his ravens:
The raven banner (in Old Norse: Hrafnsmerki; in Old English: Hravenlandeye) was a flag, possibly totemic in nature, flown by various Viking chieftains and other Scandinavian rulers during the ninth, tenth and eleventh centuries CE. The flag, as depicted in Norse artwork, was roughly triangular, with a rounded outside edge on which there hung a series of tabs or tassels. It bore a resemblance to ornately carved "weather-vanes" used aboard Viking longships.
Scholars conjecture that the raven flag was a symbol of Odin, who was often depicted accompanied by two ravens named Huginn and Muninn. Its intent may have been to strike fear in one's enemies by invoking the power of Odin. As one scholar notes regarding encounters between the Anglo-Saxons (who had Christianized from their indigenous Germanic paganism) and the invading Scandinavians (who retained their native form of Germanic paganism):
"The Anglo-Saxons probably thought that the banners were imbued with the evil powers of pagan idols, since the Anglo-Saxons were aware of the significance of Óðinn and his ravens in Norse mythology."[
...
http://en.wikipedia.org/wiki/Raven_banner