So the Lord said to him, “Bring me a heifer, a goat and a ram, each three years old, along with a dove and a young pigeon.”
Abram brought all these to him, cut them in two and arranged the halves opposite each other; the birds, however, he did not cut in half.
Genesis 15:9-10 (NIV)
The rabbis say the significance here is twofold: first, these animals represent the sacrifices acceptable to God.
Second, the cleaved animals are the nations that will later exile Israel, and that God will exact avenge.
The birds (both doves) are Israel, remaining whole, but split between two kingdoms.