What does Genesis 15:11 mean?

In response to Abram's question about how he would know that he and his offspring would possess the land, as God had promised, the Lord told Abram to bring five specific animals before Him. Abram did so and then proceeded to cut the heifer, goat, and ram (all three years old) in half and to lay each half opposite the other. He did not cut the turtledove or pigeon in half. Some scholars believe the symbolism of this ritual was meant to highlight the promise made, as if saying "may I be so destroyed if I break my part of this covenant." Others see it as an expression of unity: passing "inside" the sacrificial animals.

However, something dreadful happens that threatened to derail this covenant ritual between God and Abram. Each of the five animals chosen for the ritual were "clean" animals that would be appropriate, especially under the Law, to use as a sacrifice before God. But before the ritual is ended, a group of unclean birds of prey descend upon the remains of the animals. Abram is forced to drive them away. This seems to be an omen of the coming 400-year affliction of Abram's descendants described by the Lord's prophecy in the following verses.