He brought all these and cut them in two, placing each half opposite the other; but he did not cut up the bird.
Rashi (from Pirkei d’Rabbi Eliezer)
English: But the birds he did not divide — because the nations are compared to bulls, rams, and goats… and Israel is compared to young doves… therefore he divided the animals, a hint that the nations will gradually perish; but the birds he did not divide — a hint that Israel will endure forever.
Supplementary classical material not listed by Torah Temimah under this exact passuk.