The speaker of the poem is outside, describing the desolate winter landscape. He focuses on the barren and cold aspects as they symbolize a general despondency that the speaker feels at the end of the 19th century. He assumes that everyone in the world feels the same as he does: "And every spirit upon earth / Seemed fervourless as I."
The thrush's joyful song stands out in stark contrast to the bleak landscape the speaker has described. Despite the barren scene and what the speaker thinks is a general despair reflected in nature and in humanity at the close of the century, the thrush's song is "full-hearted" and full of "joy illimited." The speaker does not understand how the bird could sing joyfully when the world looks and feels so worthless:
So little cause for carolings
Of such ecstatic sound
Was written on terrestrial things
Afar or nigh around,
Since the bird sings joyfully amidst a hopeless seen, the speaker presumes that the bird's song indicates the possibility of hope in general and/or a hope for humanity in the next century. The speaker metaphorically says the thrush flings his soul upon the gloomy world. The bird is literally projecting his voice/song into the air as if it is giving hope to the world. The thrush's voice is its soul projected as song; a joy that potentially exists in the speaker's soul. Since the thrush is a bird, some might infer a connection to the image of the mythological phoenix which could rise from its ashes and be reborn. There is a similar theme here; a lone voice of hope rising in the air amidst a barren wasteland.