What does the bird “tell” the speaker about death?

In “Lilacs,” Whitman’s speaker transcribes the song of the hermit thrush into words. How does what the bird sings help the speaker resolve his grief for Lincoln and for the war? What does the bird “tell” the speaker about death?
https://www.poetryfoundation.org/poems/45480/when-lilacs-last-in-the-dooryard-bloomd

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