E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
means properly the function of an augur (perhaps from avium garrtus). St. Pierre says: The first navigators, when out of sight of land, watched the flight of birds, as indications of the shore, and with no other guidance discovered many new islands. From this custom (he says) arose the practice of consulting birds before entering on any important enterprise. (Studies.)