4. two birds--literally, "sparrows." The Septuagint, however, renders the expression "little birds"; and it is evident that it is to be taken in this generic sense from their being specified as "clean"--a condition which would have been altogether superfluous to mention in reference to sparrows. In all the offerings prescribed in the law, Moses ordered only common and accessible birds; and hence we may presume that he points here to such birds as sparrows or pigeons, as in the desert it might have been very difficult to procure wild birds alive.
      cedar-wood, and scarlet, and hyssop--The cedar here meant was certainly not the famous tree of Lebanon, and it is generally supposed to have been the juniper, as several varieties of that shrub are found growing abundantly in the clefts and crevices of the Sinaitic mountains. A stick of this shrub was bound to a bunch of hyssop by a scarlet ribbon, and the living bird was to be so attached to it, that when they dipped the branches in the water, the tail of the bird might also be moistened, but not the head nor the wings, that it might not be impeded in its flight when let loose.

JFB.