Explain Hardy as a nature poet in regards to "The Darkling Thrush."

As a Naturalist, Thomas Hardy employs nature to convey
both mood for his works and to convey the Naturalistic indifference of the universe. 
For instance, his poem entitled "Neutral Tones" he and his love stand by a pond on a
winter day, a pond "edged with grayish leaves," that reflects the tone of this poem. 
This dismal tone underscores the remembrances of the speaker about his former love as
their relationship ended:  "The smile on your mouth was the deadest thing."  With the
sun described as "as thou chidden of God" and the "starving sod," nature conveys the
dismal mood of the speaker.

In his poem "The Darkling
Thrush," Hardy writes that

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The land's sharp features seemed to
be

The Century's corpse
outleant,

His crypt the coudy
canopy,

The wind his
death-lament.

But, just when
the speaker believes that nature shares his mood, a darkling thrush appears and
sings

His
happy good-night air

Some blessed Hope, whereof he
knew

And I was
unaware.

So, it really is an
indifferent universe that, in its caprice that the speaker does not understand, has a
cheerful song.