What role did the the turtle dove and pideon have in Abraham's sacrifice?

Why is it said of Abram in Genesis 15:9-10, “The birds divided he not”?

Why were the animals divided, but not the birds?

We see in verse 9 that, preparatory to making a covenant with Abram, the Lord God directed him to take certain sacrificial victims—a heifer of three years old, a she-goat of three years old, a ram of three years old, a turtle dove and a young pigeon. On the slain carcases of these the covenant was to be made. Evidently God had given general directions as to sacrifices and the clean animals to be offered up, as we gather from the knowledge of such things displayed, for example, by Noah, who knew the animals that were clean, that is, fit for sacrifice, and unclean— unfit (see Genesis 7:2, 3); and the same with the fowls—of the unclean two would suffice, but the clean were to go in “by sevens”—as these latter would be needed for sacrifice (Genesis 8:20). No doubt, too, the order to be followed was divinely revealed. The very word for covenant, “berlth, from an unused root, “barath” means “to cut, to hew”— that is, the victims were divided—he who made the covenant had to pass between the Victims. In Jeremiah 34:19, those who made the covenant passed between the calves—all typical of Christ, the Mediator of the New Covenant.

According to Hebrews 9:17 the covenant is always made on the ground of dead things (i.e. sacrifices). I am convinced that the idea at the back of this whole passage is that of a covenant, not a testament or will in our modern sense, as the next verse seems clearly to show. If we turn to Exodus 24: , there is no thought of a will being made, but of a covenant being established between Jehovah and Israel. How then could the second covenant be a testament? The R.V., although somewhat hesitatingly retaining “covenant” up to verse 15, translates verse i: “For a testament is of force where there hath been death” (margin, “over the dead”). The word, “dead,” is plural, and therefore must, one would judge, refer to the covenant victims, rather than to the testator of a will—who is a single individual. I suppose that the reason Abram did not divide the birds is that they formed one offering in two parts-_one of which was placed on each side, representing a cut victim. To those who have a deeper appreciation of the work of Christ, the Word divides the victim even between the . . . joints and
marrow (Hebrews 4:12).

W.H.