What is the distinct quality of the thrush's song?

The thrush in Hardy's poem is singing cheerfully, taking no cognizance of the bleak weather and the winter chill:

At once a voice arose among
The bleak twigs overhead
In a full-hearted evensong
Of joy unlimited.

The irony the speaker finds in the bird's joy is emphasized by the fact that the thrush itself is "aged, frail, gaunt and small."
The setting, which is cold, desolate, and approaching evening in winter, is a metaphor for the bleakness the speaker sees in his own—and mankind's—forlorn hopes in a hostile universe. He regards the bird's apparent joy as evidence the bird knows of some positive factor in the world of which he himself is unaware.
The poem is expressive of Hardy's intense pessimism that we see in his verse as a whole and in his novels. But it is also part of a long literary tradition of the poet bonding with a bird-figure and seeing the bird as a kind of counterpart to his or her own thought, or as a foil, contrasting with or expressing a different view from the speaker. Compare "The Darkling Thrush" with other such poems: for instance, Keats's "Ode to a Nightingale." In the latter the speaker contemplates suicide, while the bird is "pouring forth [its] soul in ecstasy." In Robert Herrick's "To Robin Redbreast," the speaker contemplates his own death and the robins (with a pun on the poet's own nickname) building his grave for him. One might ask what the special or unique character of Hardy's poem is—if there is such uniqueness—that might set it apart from these and other earlier, famous bird-inspired poems.