Augury — from the Latin avis (bird) and garrire (to talk) — was, in Rome, a formal state institution. The College of Augurs was one of the most important religious bodies in the Roman Republic and Empire; no major state action — the founding of a city, the beginning of a military campaign, the election of magistrates — could proceed without consulting the augurs and receiving their interpretation of the bird signs.
Roman augury had a precise technical vocabulary and methodology. The augur defined a sacred space (the templum) and observed birds entering, leaving, flying over or calling within it. The species, direction of flight, calls made and behavior all carried specific meanings. Eagles and vultures were the most significant birds; their appearance on the right (the lucky side) was favourable, on the left unfavourable. Chickens were also consulted — their appetite before a battle was a reliable oracle in Roman eyes.
But Roman augury was the formalisation of a far more ancient and universal practice. Greek oracles included bird observation. Mesopotamian divination texts include sections on bird omens. The ancient Celts, Norse, Egyptians and virtually every other ancient culture developed their own systems for reading the messages carried by birds and other animals.