Mundaka Upanishad, Third Chapter, Canto-1, verses 1-3 (translation):
Two birds that are ever associated and have similar names, cling to the same tree. Of these, one eats the fruit of divergent tastes, and the other looks on without eating. On the same tree, the individual soul remains drowned (i.e. stuck), as it were, and so it moans, being worried by its helplessness. When it sees thus the other, the adored Lord, and His glory, then it becomes liberated from sorrow. When the seer sees the Purusha – the golden hued, creator, Lord, and the source of the inferior Brahman – then the illumined one completely shakes off both merit and demerit, becomes taintless, and attains absolute equality .
The bound bird is the individual soul who is ignorant of his/her true nature, the free one is God, the tree is the dwelling place (i.e., body-mind complex), fruits – sweet (pleasures) and bitter (pain) – are results of one’s actions (karma), helplessness signifies the travails of being in ignorance, the “seeing” towards Purusha (or the free bird) implies turning attention towards God and doing spiritual practice (sadhana) to attain the Lord. Having attained the goal, the sense of individuality is lost completely and one is beyond the effects of karma (both merit and demerit).
I’ve attempted to depict this symbolism in poetry as follows:
One bound to the world and the other eternally free.
. Eight Upanishads with the commentary of Sankaracharya (Vol. 2) – Translated by Swami Gambhirananda