At the beginning of "The Darkling Thrush," the speaker presents a rather somber, melancholy, and wintry scene. He says that the "frost was spectre-grey" and that the "winter's dregs made desolate / The weakening eye of day." The season of winter connotes cold, darkness, and death. The word spectre also suggests that the speaker feels haunted by this cold, darkness, and death. The image of the "weakening eye of day" suggests that the daylight is being overwhelmed by the darkness.
In the second stanza, the speaker compounds this impression of desolation and misery. He personifies the century that is drawing to a close as a "corpse outleant," and he describes the dark clouds overhead as the "crypt" in which that corpse is laid.
In the third stanza, the speaker hears "a voice" that he describes as "full-hearted" and full of "joy illimited." This voice belongs to an "aged thrush, frail, gaunt and small." Hearing the beautiful and joyous voice of the bird gives the speaker a momentary, fleeting cause for hope. Hearing the bird's song, the speaker is able to believe, even if only for a brief moment, that there is life and hope in this otherwise dark and dreary world.
The fact that the thrush is "aged ... frail, gaunt and small," however, indicates that the life and hope symbolized by the bird will only be temporary. This is also emphasized by the title of the poem. The -ling suffix in "darkling" implies something that is small and getting smaller. The word darkling to describe the thrush also suggests that even the thrush, with the hope which it symbolizes, is being overwhelmed and consumed by the pervasive, ubiquitous darkness of the wintry scene.