Albert Barnes Commentary Job 28:21

"Seeing it is hid from the eyes of all living, And kept close from the birds of the heavens." — Job 28:21 (ASV)

It is hid from the eyes of all living - That is, of all people, and of all animals. Man has not found it by the most sagacious of all his discoveries, and the keenest vision of beasts and birds has not traced it out.

And kept close - Hebrew “concealed.”

From the birds of the air - Compare the notes at Job 28:7. Umbreit remarks, on this passage, that there is attributed to the birds in Oriental countries a deep knowledge, and an extraordinary gift of divination, and that they appear as the interpreters and confidants of the gods.

One cannot but reflect, says he, on the personification of the good spirit of Ormuzd through the birds, according to the doctrine of the Persians (Compare Creutzer’s Symbolik Thes 1. s. 723); upon the ancient bird-king (Vogelkonig) Simurg upon the mountain Kap, representing the highest wisdom of life; upon the discourses of the birds of the great mystic poet of the Persians, Ferideddin Attar, etc.

Among the ancient Greeks and Romans, also, a considerable part of their divinations consisted in observing the flight of birds, as if they were endowed with intelligence, and indicated coming events by the course which they took. Compare also Ecclesiastes 10:20, where wisdom or intelligence is ascribed to the birds of the air: Curse not the king, no, not in thy thought; and curse not the rich in thy bed-chamber: for a bird of the air shall carry the voice, and that which hath wings shall tell the matter.