Discuss "The Darkling Thrush" as an elegy for the troubled nineteenth century.

Hardy wrote the poem just a couple of days before the beginning of what, for many people, would've been the official start of the twentieth century. (Indeed, the poem was originally entitled "The Century's End: 1900.") It should come as no surprise, then, that so many literary critics have seen "The Darkling Thrush" as a lament for a passing era.
The poem is suitably set at dusk—during the "weakening eye of day." The ominousness with which the speaker anticipates the new century is heightened by the "cloudy canopy" that acts like a crypt to the dying days of the nineteenth century. The death of the old—"The Century's corpse"—clearly disturbs the speaker, especially as he seems to have no idea of what will follow.
The imminent dawn of a new century is not a cause for celebration, no time for joyous "carolings." Even the hearty song of the thrush seems wasted, coming as it does against the backdrop of gathering gloom. This song is truly a lament for a vanishing era: not, as may be hoped, a herald of a bright new dawn.
https://www.poetryfoundation.org/poems/44325/the-darkling-thrush