Upon the same tree there are two birds, one on the top, the other below. The one on the top is calm, silent, and majestic, immersed in his own glory; the one on the lower branches, eating sweet and bitter fruits by turns, hopping from branch to branch, is becoming happy and miserable by turns. After a time the lower bird eats an exceptionally bitter fruit and gets disgustful and looks up and sees the other bird, that wondrous one of golden plumage, who eats neither sweet nor bitter fruit, who is neither happy nor miserable, but calm, Self-centred, and sees nothing beyond his Self.
The lower bird longs for this condition but soon forgets it, and again begins to eat the fruits. In a little while, he eats another exceptionally bitter fruit, which makes him feel miserable, and he again looks up, and tries to get nearer to the upper bird. Once more he forgets and after a time he looks up, and so on he goes again and again, until he comes very near to the beautiful bird and sees the reflection of light from his plumage playing around his own body, and he feels a change and seems to melt away; still nearer he comes, and everything about him melts away, and at last he understands this wonderful change. The lower bird was, as it were, only the substantial-looking shadow, the reflection of the higher; he himself was in essence the upper bird all the time.
This eating of fruits, sweet and bitter, this lower, little bird, weeping and happy by turns, was a vain chimera, a dream: all along, the real bird was there above, calm and silent, glorious and majestic, beyond grief, beyond sorrow. The upper bird is God, the Lord of this universe; and the lower bird is the human soul, eating the sweet and bitter fruits of this world. (II – 394-95)
Swamiji often used to quote this classical example from Mundaka Upanishad to illustrate the relation between Jeevatma and Paramatma.