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, who would attempt to destroy the nations, 1 but whom HaSh-m would stop, as this will only become achievable with the advent of Mashi'ach.
Targum Yonasan: On the contrary, it hinted 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.
Ramban: In similar vein, it hints at the nations of the world, who would try to block the Korbanos, but whom Avraham's descendants would chase away.
"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 HaSh-m, and no others have a portion in it.