A crow's nest is traditionally a
platform high up on the
mast of a ship, occupied by one or two unlucky
sailors whose job it is to
freeze their
hindquarters off while looking around for any
suspicious entities, in or out of the water. It's generally
a bad thing to
sleep while on duty in a crow's nest, because otherwise you might miss something (the
Titanic is probably a good example, though that may not have been so much of a timing issue). However, crow's nests have been made
virtually obsolete with the
technological capabilities of
modern naval craft. If anything, they are there for the sake of
tradition, and are rarely manned (
though there may be military standards, so feel free to correct me).
A note from karma_debt: Watches for all sorts of things (even archaic things) are still manned by most Naval ships, and other militaristic things... the military thrives on silly, archaic, yet traditional things.
Source: Dictionary.com