A songbird is any one of the 4000 species of perching birds with vocal organs developed to produce diverse and elaborate bird song. The bird song is used to communicate sexual intentions and territorial presence.
The rare song bird in the golden cage is a symbol for the trapped human soul, restricted by the desires and longings of the material world, ensnared in the cycle of birth and death, isolated and separated from its Cosmic Self, from the Supreme Being, the Brahman.
The death of the song bird further signifies the futility of its existence, of the failure to free oneself from samsara, of the tragedy of birth and death and ensnarement in the golden cage. The death of the birth fills Siddhartha with a sense of loss because he identifies with the purity and goodness of the songbird, the rarity of his Self, that he has allowed to dissipate, frittered away in the lust for wealth, wine and women.