One important theme of "Birches" is how Frost uses his poetic imagination to transcend the limits of the real world. He rejects the true reason the birches have been bent over in favor of his own fanciful explanation. On some level, he is claiming that this act of the imagination embodies a larger "truth" and is a worthy task, one that must be made with great care and diligence.
2]The theme of Youth
Youth, like death, is a constant backdrop for many of Frost's poems. The speaker of "Birches" never sees a boy or comes across one. He only imagines one, and the boy that he does imagine is himself at a younger age. The boy seems to be similar to William Wordsworth and Walt Whitman's portrayals of boys. These boys have their own rules and wisdom that they can pass on to the older men and women around them. They are ready for adventures in nature and represent the wild, untamed state of "man" that remains good and moral even though no one is there to govern him.
3] The theme of Spirituality
Robert Frost is not the kind of poet to insert religious imagery into his poems. A subtle Christian allusion is rare. However, the poet writes a lot of meditations on life and death, so that always brings in spiritual questions. In "Birches," Frost mentions "heaven" twice. Notice how it is always with a lower-case h and is more suggestive of the sky than paradise. The poem could be read as an allegory, but it's a little too skeptical for that.
4]The theme of isolation
As with much of Frost's poetry, "Birches" creates a mood of loneliness and isolation. Some factors that contribute to the mood include the winter weather, which seems to cut the speaker off from other people, and the speaker's discussion of the boy growing up on an isolated farm. The speaker's loneliness may be the result of adult concerns. and considerations.