What is The Cocoon by Robert Frost theme?

Throughout the course of the poem, "My Butterfly" by Robert Frost, the author uses the theme of death and sorrowfulness; a feeling that is emitted during the loss of a loved one; in this case, Frost's father. As the speaker says in the 8th stanza of the poem, "I found that wing broken today ! | For thou art dead, I said. | And the strange birds say. | I found it withered leaves | Under the eaves.", the quote illustrates how the speak acknowledges the death of someone after the "wing [is] broken".

(PS. This was my homework for my English class; editing is much appreciated, Thank you !)