Tree don't care what the little bird sings
Trees are incapable of thoughts or feelings, and therefore a symbol of the indifference of the world itself. The world doesn’t care how beautiful or clever human beings think they are; it will still let them die. The trees don’t care how beautifully a bird can sing.
Also, the “tree” possibly represents the human race, and the “little bird” represents a person. Cave is essentially stating that one person’s opinion won’t make a bit of difference, as it will be drowned out by millions of other voices.
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