Thomas Hardy uses the thrush in his poem "The Darkling Thrush" to signify the appearance of hope in an otherwise gloomy and harsh environment. "The Darkling Thrush" is all about Hardy's contemplations of the end of things: the end of the year, the end of the century; his language is dark, pessimistic, and gloomy. However, in stanza three, both tone and mood suddenly shift as the speaker hears:
"At once a voice arose among
The bleak twigs overhead
In a full-hearted evensong
Of joy illimited;" (17-20)
The thrush's sudden appearance changes the speaker's outlook. This bird may look beat-up and a little decrepit, but the bird pours his heart out in song, uncaring of the surrounding gloom. Even the hardened and glum speaker recognizes that the birds' cheerful song belies his hostile environment, making the speaker wonder if the bird knows something he does not, some reason to be that cheery.
The thrush symbolizes unlooked-for optimism and a reason to carry on even through dark, uncertain times.