What is the significance of the birds of prey that descended upon the pieces, and of Avraham chasing them away?
Rashi: This hinted at David ha'Melech, who would attempt to destroy the nations, 1 but whom Hashem would stop, as this will only become achievable with the advent of Mashi'ach.
Ramban: It hints at the nations of the world, who would try to block the Korbanos, but whom Avraham's descendants would chase away.
Targum Yonasan: It hints at the nations of the world, who would come to plunder the property of Avraham's descendants, but who would not succeed, due to the merits of Avraham.
"Birds of prey descended on the carcasses, and Avraham drove them back." From here we learn that one who offers a Korban must stand by as it is offered (Ma'amados). What does this act show?
Maharal (Gevuros Hashem Ch. 8, p. 47): Ma'amados show that the Avodah is solely for Hashem, and no others have a portion in it.