Augury was a Greco-Roman religious practice of observing the behavior of birds, to receive omens. When the individual, known as the augur, read these signs, it was referred to as "taking the auspices". "Auspices" (Latin: auspicium) means "looking at birds". Auspex, another word for augur, can be translated to "one who looks at birds". Depending upon the birds, the auspices from the gods could be favorable or unfavorable (auspicious or inauspicious). Sometimes politically motivated augurs would fabricate unfavorable auspices in order to delay certain state functions, such as elections. Pliny the Elder attributes the invention of auspicy to Tiresias the seer of Thebes.

Over the development of the Roman empire, the definition of augury broadened to include other forms of divination. Haruspicy—the examination of animal entrails—was learned from the Etruscans. The Etruscan practice of observing thunder and lightning was also adapted. In Cicero’s time, the augurs had mostly switched from using the flight of birds to haruspicy for public divination.

This type of omen reading was already a millennium old in the time of Classical Greece: in the fourteenth-century BC diplomatic correspondence preserved in Egypt called the Amarna correspondence, the practice was familiar to the king of Alasia in Cyprus who needed an "eagle diviner" to be sent from Egypt. This earlier, indigenous practice of divining by bird signs, familiar in the figure of Calchas, the bird-diviner to Agamemnon, who led the army (Iliad I.69), was largely replaced by sacrifice-divination through inspection of the sacrificial victim's liver—haruspices—during the Orientalizing period of archaic Greek culture. Plato notes that hepatoscopy held greater prestige than augury by means of birds.

One of the most famous auspices is the one which is connected with the founding of Rome. Once the founders of Rome, Romulus and Remus, arrived at the Palatine Hill, the two argued over where the exact position of the city should be. Romulus was set on building the city upon the Palatine, but Remus wanted to build the city on the strategic and easily fortified Aventine Hill. The two agreed to settle their argument by testing their abilities as augurs and by the will of the gods. Each took a seat on the ground apart from one another, and, according to Plutarch, Remus saw six vultures, after which Romulus saw twelve. The two clashed over whether the preference of the gods was indicated by Remus seeing vultures before Romulus did, or by Romulus seeing twelve vultures while Remus saw six. Vultures were pre-eminent in Roman augury, furnishing the strongest signs an augur could receive from a wild bird. They were subject to protective taboos and also called sacred birds.

According to unanimous testimony from ancient sources, the use of auspices as a means to decipher the will of the gods was more ancient than Rome itself. The use of the word is usually associated with Latins as well as the earliest Roman citizens. Though some modern historians link the act of observing Auspices to the Etruscans, Cicero accounts in his text De Divinatione several differences between the auspicial of the Romans and the Etruscan system of interpreting the will of the gods. Cicero also mentions several other nations which, like the Romans, paid attention to the patterns of flying birds as signs of the gods' will but never mentions this practice while discussing the Etruscans.

Though auspices were prevalent before the Romans, Romans are often linked with auspices because of their connection to Rome's foundation and because Romans established rules for the reading of auspices that helped keep it an essential part of Roman culture. Stoics, for instance, maintained that if there are gods, they care for men, and that if they care for men they must send them signs of their will. Even the Philistines practiced augury as far back as 740 BC and c. 686 BC as declared by Isaiah 2:6 in the Hebrew Bible. Yet augury was first systematized by the Chaldeans according to the Jewish Encyclopedia.

In ancient Rome, the appointment and inauguration of any magistrate, decisions made within the people's assembly and the advancement of any campaign, always required a positive auspicium. This auspicium was only considered effective for either one day or the period it takes for the action to be completed.

During Octavian's first consulship in 43 B.C.E., the positive auspicium corresponded to the spotting of twelve vultures, similar to Romulus. Unlike in Greece where oracles played the role of messenger of the gods, in Rome it was through birds that Jupiter's will was interpreted. Auspices showed Romans what they were to do, or not to do; no explanation for the decision was given except that it was the will of the gods. It would be difficult to execute any public act without consulting the auspices.