Augustine of Hippo : Here also, in fine, a symbol was given, consisting of these animals: a heifer, a she-goat, a ram and two birds, a turtledove and pigeon, that he might know that the things which he had not doubted should come to pass were to happen in accordance with this symbol. The heifer may be a sign that the people should be put under the law, the she-goat that the same people were to become sinful, the ram that they should reign. Perhaps these animals are said to be of three years old for this reason: that there are three remarkable divisions of time, from Adam to Noah, and from him to Abraham, and from him to David. David, on the rejection of Saul, was first established by the will of the Lord in the kingdom of the Israelite nation. In this third division, which extends from Abraham to David, people grew up as if passing through the third age of life. Or perhaps it may be that they had some other more suitable meaning. Still I have no doubt whatever that spiritual things were prefigured by them as well as by the turtledove and pigeon. — City of God 16.24 Bede : The Lord replied: "Take for me a three-year-old heifer, a three-year-old goat, and a three-year-old ram, also a turtledove and a pigeon." He took all these and divided them in half, and placed each half opposite the other, but he did not divide the birds. For the heifer may signify the people under the yoke of the law; the goat the same people who would be sinful, the ram the same people who would also reign. Therefore, these animals are called three because in the third age of the world that people matured and entered the land of promise. The first age is from Adam to Noah, the second from Noah to Abraham himself, the third from Abraham to David; in which the people were saved from Egyptian servitude and transferred to the land of promise. Or if these signify something else more fitting, I would by no means doubt that the spiritual are prefigured in the addition of the turtledove and the pigeon; and thus it is said: "But he did not divide the birds," because the carnal are divided among themselves, but the spiritual by no means, whether they remove themselves from the busy dealings of men like the turtledove, or dwell among them like the pigeon; yet both birds are simple and harmless, signifying that in the very Israelite people, to whom that land was to be given, there would be undivided peoples of promise and heirs of the kingdom remaining in eternal happiness. — Commentary on Genesis (Hexaemeron) Caesarius of Arles : Therefore the heifer, the she-goat and the ram of three years, as also the turtledove and the pigeon, presented a type of all nations. They were described as of three years, because all the nations were to believe in the mystery of the Trinity. Now the entire Catholic church has not only spiritual members but carnal ones also, for although some say they believe in the Trinity, they are nevertheless carnal because they neglect to avoid sins and vices. Since there are spiritual souls with the carnal ones, for this reason the turtledove and pigeon were added. In the latter, spiritual people can be meant, but in those other three animals carnal people are understood. — SERMON 82.1