Jamieson, Fausset and Brown on Isaiah 18:6

6. birds ... beasts--transition from the image "sprigs," "branches," to the thing meant: the Assyrian soldiers and leaders shall be the prey of birds and beasts, the whole year through, "winter" and "summer," so numerous shall be their carcasses. Horsley translates the Hebrew which is singular: "upon it," not "upon them"; the "it" refers to God's "dwelling-place" (Is 18:4) in the Holy Land, which Antichrist ("the bird of prey" with the "beasts," his rebel hosts) is to possess himself of, and where he is to perish.