(Verses 10, 11.) And he placed them opposite each other: but he did not separate the birds. However, birds descended upon the carcasses and the divisions thereof, and Abram drove them away. It does not pertain to the present work to explain the sacrament. We only say this, because the Hebrew version has for these words: And birds descended upon the carcasses, and Abram drove them away. For he deservedly often delivered Israel from narrow straits.
The fowls coming down on the divided carcasses represent nothing good but [rather] the spirits of this air, seeking some food for themselves in the division of carnal people. But that Abraham sat down with them signifies that even amid these divisions of the carnal, true believers shall persevere to the end. With the going down of the sun great fear fell upon Abraham and a horror of great darkness. This signifies that about the end of this world believers shall be in great perturbation and tribulation, of which the Lord said in the Gospel, “For then shall be great tribulation, such as was not from the beginning.”
And birds of prey came down upon the carcasses, and Abram drove them away. These divided birds, which descended upon the carcasses, do not indicate anything good, but rather certain spirits of this air seeking their sustenance from the division of the carnal; or certainly the carnal adversaries of the same people, who, according to the counsel of Balaam the diviner, were seeking an opportunity for victory from their crimes, concerning whom the prophet says: "Our pursuers were swifter than the eagles of the heavens" (Lamentations 4:19).
But Abram drove them away, because by his merits Israel was often delivered both from the distress of temporal evils and from the snares of evil spirits.
[AD 420] Jerome on Genesis 15:10-11