classic waka stream

How rare a thing—that
from its lodging among flowers,
passing tree to tree,
the warbler should come at last
to the old nest in the valley

Meaning
How rare a thing—that from its lodging among the flowers, passing from tree to tree, the warbler should come at last to the old nest in the valley.
Commentary
Part One: Hatsune

 Toward dusk on New Year's Day Genji went across to the lady of Akashi. He had only to push open the door of the gallery for the perfumes within the blinds to come out on the air, elegant, and of a distinction beyond anything. She herself was not to be seen. Looking about, he found practice sheets scattered near the inkstone, and took them up to read. A brazier of choice make, with the incense called Jiju burning in it; a koto set on a brocade cushion from China—everything spoke of taste. Her hand ran neither too far into cursive nor into display, but quietly and well. Having read her daughter's answer with rare pleasure, she had written this among snatches of old poems. Genji picked it up and smiled. Presently she came forward on her knees, keeping herself, even so, modestly in the background, and he thought again that she was not as others are; and though he feared what a stir it might make so early in the year, he stayed the night there.

 "The lodging among flowers" is Murasaki's quarter, where the girl lives; "the old nest in the valley" is her own. "Passing tree to tree" is the warbler's flitting from branch to branch, hinting at a child held by Genji and raised by Murasaki. "Mezurashi" is rare, and glad. A warbler that has left the valley for the high branches and returns to its old nest is a figure of spring poems from the Kokinshu on; here it falls exactly across her own case. That the poem was written among practice sheets rather than sent as an exchange is what makes it ache. Beside it she had set the line "since my house stands by the hill where the plum is in bloom"—consoling herself that her daughter is in the house of flowers, where word may always be had.
Author
Murasaki Shikibu
Source
Genji Monogatari
Other