Why did Thomas Hardy write "The Darkling Thrush" and what is its theme?

Thomas Hardy (1840-1928) wrote "The Darkling Thrush" on 31st December 1899. It was first published in "Graphic" with the subtitle "By the Century's Deathbed," and was later published in "The London Times" on New Year's Day 1901.

The poem could be understood as symbolically marking the end of a century and the beginning of a new one. It must be remembered that Queen Victoria died on 22nd January 1901 thus marking the end of a great era, 'The Victorian Age' in English history, literature and culture. Britain's supremacy as a world super power was beginning to be challenged by Germany and the two world wars were imminent. This is the socio-historical background of the publication of this poem.

The first two stanzas of the poem describe the bleakness  of the countryside on a dark, cold wintry evening. The entire  countryside seemed to represent the tomb of the 19th century and the overall mood and atmosphere was one of intense gloom:

The land's sharp features seemed to be
The Century's corpse outleant,     
His crypt the cloudy canopy,
The wind his death-lament.
The ancient pulse of germ and birth
Was shrunken hard and dry,
And every spirit upon earth     
Seemed fervourless as I.

Just when Hardy thought that there was nothing to cheer about he heard the song of a thrush which seemed to give him some hope of a bright future in the coming 20th century. However, Hardy ends the poem on a skeptical note. He says that although the thrush sings cheerfully he is not sure what is the theme of its song  or why it is singing so cheerfully. He says that he cannot see any reason for the cheerfulness of the bird's song in the gloomy and desolate  scene around him:

"So little cause for carolings     
Of such ecstatic sound
Was written on terrestrial things
Afar or nigh around,
That I could think there trembled through
His happy good-night air     
Some blessed Hope, whereof he knew
And I was unaware."

When Hardy wrote this poem he was in sixties, and his novel "Jude the Obscure" (1895) had been criticized harshly and had been burnt in public with the result that Hardy stopped writing novels and turned to writing poetry. The "aged thrush" could thus be seen to represent Hardy himself with the thrush becoming the objective correlative signifying not only the uncertain future of the new 20th century but also the uncertain future of Hardy as a poet:

An aged thrush, frail, gaunt, and small,
In blast-beruffled plume,
Had chosen thus to fling his soul
Upon the growing gloom.