avis

Avis, avis, f., third declension i-stem: genitive plural avium, accusative plural aves, ablative singular ave. Feminine even when the bird is obviously male, which catches people out in agreement.

Beyond feathers, avis is a technical term of Roman religion. Birds were how the gods answered a question, so an avis is a sign, and the standing idiom uses the plural: bonis avibus, "under good birds", meaning with favourable omens, and malis avibus for the opposite. The official who watched them was the auspex, avis plus specio, "bird-watcher", from which auspices and auspicious; augur is probably from the same nest. English aviary, aviation and avian are later borrowings straight from the noun. Do not confuse it with ovis, "sheep", which differs by one letter and never flies.