John Calvin Commentary Genesis 15:11

"And the birds of prey came down upon the carcasses, and Abram drove them away." — Genesis 15:11 (ASV)

And when the fowls came down. Although the sacrifice was dedicated to God, yet it was not free from the attack and violence of birds. Neither are the faithful, after they are received into God's protection, so covered by his hand that they are not assailed on every side, since Satan and the world do not cease to cause them trouble. Therefore, in order that the sacrifice we have once offered to God may not be violated but may remain pure and uninjured, contrary assaults must be repulsed, with whatever inconvenience and toil.