Gávea: A “crow’s nest” was a cylindrical structure atop the mainmast - the highest point on a sailing vessel.
Back in the times of the early ocean voyages, it was from this platform, usually barrel shaped, that lookouts would scour the horizon for other ships or signs of land.
From the crow’s nest, the historical accounts tell us, sailors would holler Land ahoy! on first spotting signs of land on the horizon.