a bird: any flying creature, treated as one class of animal
This word comes from the verb "to fly," so at root it names "the flying one," a winged creature. In practice it is simply the ordinary word for birds, and in the Gospels it almost always appears in the set phrase "the birds of the air" (or "of the sky"): the birds that eat the scattered seed in the parable of the sower, the birds that nest in the branches of the grown mustard plant, the birds that have nowhere Jesus can lay his head to compare with, and the birds God feeds without their sowing or reaping.
Because the word groups every winged creature into a single category, it also shows up in the sweeping lists that sort all living things: Peter's vision of the sheet holds four-footed animals, reptiles, and birds; the idolaters of Romans exchange God's glory for images of people, animals, reptiles, and birds; and James says every kind of beast and bird, reptile and sea creature, can be tamed. There it stands beside the land animals, the crawling things, and the creatures of the sea as one of the great divisions of the animal world.
Greek has more specific words when a particular bird is in view, such as a hen or mother bird (ὄρνις), but this is the general term, the equivalent of saying "birds" without naming which kind.