How does the third staza of The darkling thrush strike a different tone

There is a sense of happiness and beauty amidst the gloom.

A sudden hurling out of a song by a thrush is heard, rising from the depth of winter. A aged bird, that looked thin and exhausted, survives the dreariness of winter and sings a beautiful song of hope amidst the growing gloom.While Other stanzas present a sad face.