Because of its themes of renewal, life in darkness, and finding joy when circumstances seem dire, "The Darkling Thrush" contains many examples of powerful and expressive imagery. One good example comes in the first lines:
I leant upon a coppice gate
When Frost was spectre-gray,
And Winter's dregs made desolate
The weakening eye of day.
"Spectre-gray" likens the frost and dusk of the day to a spectre, or ghost. This gives it a foreboding aspect, as if the frost is a malevolent force instead of simple cold. As the day ends, its "eye" (the sun) weakens; the spectre of the frost is fighting against the day's eye and forcing it to close.
In the next verse, the narrator refers to the land as seemingly-dead, with:
His crypt the cloudy canopy,
The wind his death-lament.
Both of these lines show the world as a living creature that is being slowly killed by the darkening days and encroaching frost. The clouds overhead are the canopy adorning a crypt, while the wind sings a funeral dirge in the world's honor.
An aged thrush, frail, gaunt, and small,
In blast-beruffled plume,
Had chosen thus to fling his soul
Upon the growing gloom.
(Hardy, "The Darkling Thrush," en.wikipedia.org)
The thrush is the spirit of life, the last color in the graying world. It is tiny, especially compared to the enormity of Winter, but it gives all of itself that it has. By "flinging his soul" into the night and celebrating his own small life, the thrush gives hope to others and allows the narrator to see that there might be something further down the road; while the Winter is harsh, it is not the literal end of the world, as the thrush has now affirmed.