The story
From the habit of crows building nests high up in trees.
Etymology from Wiktionary, CC BY-SA 4.0.
crow's nest
Today it means: A small open-top shelter, originally a cask, on the top of the foremast, large enough to accommodate a lookout. It was used by whalers to watch for a blow (spout), or in icebound waters to seek a channel. Full definition →