Why does the poet disbelieve the bird's "carolings"?

Amidst the deafening silence and graveyard-like doom and despondency, the speaker hears a strange loud sound emanating from the background of the dry lifeless twigs and stems of the nearby tree. It catches the speaker by surprise. The strange sound seems to announce that hope and health is returning. The speaker discovers that an old, enfeebled thrush with frayed plumes is crying out. But, the intent of the thrush in making the cry, when all life forms are on the verge of eternal silence, baffles the speaker. This makes the speaker wonder as to what had given the bird hope and positivity.