Of the ferryman across the "Waters of Death" there is no trace in the Old Testament. Spirits are supposed rather to "fly away" to their abode (Ps. 90:10). The bird-like form assumed by the soul for its journey was a wide-spread belief of antiquity, and appears probably in the word "twitter" that is used of the voice of ghosts in Isa. 8:19; 29:4. This idea was not unknown to the Babylonians. In Ishtar's Descent (obv. 10) we read of the shades, "They are clothed like a bird in a garment of feathers."
Lewis Bayles Paton, "The Hebrew Idea of the Future Life. III. Bablyonian Influence in the Doctrine of Sheol", The Biblical World, Vol. 35, No. 3 (Mar., 1910), p. 164.
Isa. 8:19: οἱ κενολογοῦνται "the twitterers", lit. "the empty-talkers"