Origin and history of auspex

"ancient Romans functionary whose duty was to observe and interpret auspices, or reputed natural signs concerning future events," 1540s, from Latin augur, a religious official in ancient Rome, perhaps (de Vaan) originally meaning "an increase in crops enacted in ritual," in which case it probably is from Old Latin *augos (genitive *augeris) "increase," and is related to augere "increase" (from PIE root *aug- (1) "to increase").

The more popular theory is that it is from Latin avis "bird," because the flights, singing, and feeding of birds were important objects of divination (compare auspex). In that case, the second element would be from garrire "to talk." Related: Augural; augurial.

These auspices were studied, with a fixed ceremonial, in the following classes of phenomena: (1) signs from the heavens, including thunder and lightning, and other meteorological manifestations; (2) signs from the direction of flight or the various cries of birds; (3) signs from the manner of eating of domestic hens kept for this purpose; (4) signs from the movements and attitudes of animals; (5) evil omens from various fortuitous incidents, such as the fall of any object, the gnawing of a mouse, the creaking of a chair, etc., occurring during the augural ceremonies or when these were about to begin. [Century Dictionary]

plural (and now the usual form) of auspice (1530s), "observation of birds for the purpose of taking omens," from French auspice (14c.), from Latin auspicum "divination from the flight of birds; function of an auspex" (q.v.).

The meaning "any indication of the future (especially favorable)" is from 1650s; it is attested earlier (1630s) in extended sense of "benevolent influence of greater power, influence exerted on behalf of someone or something," originally in the expression under the auspices of.