Charles Ellicott 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 ... — Hebrew, And the birds of prey came down upon the carcases, and Abram scared them away. Had there been a sacrifice, the fire would have kept the vultures from approaching; but the bodies lay exposed, and Abram therefore kept guard over them, so that the purpose of the ceremonial would not be frustrated by any lack of respect shown to the outward symbols.