What is the "nature of mortality", or atleast what is it in this poem?

I am so glad you have returned to school and are taking this class. You too shall write, inspire and empower. Life will get better and better as you follow that little bird before you, who stops putting a tree before you...I see your returning to school as a tree for you to explore the branches, climb high, stretch your limbs and eat the fruit until the little bird calls you to proceed to him and follow that little fellow. I love Birches which says to me some times we may wish to zone out, get depressed , retreat,be silly and want to fly into the wind, perhaps to die, but Frost says being a part of this world, and all its wonder, worry, sorrow, etc. it is the right place for "me" . Blessings to you as your build bridges within your self and soar. Thanks for sharing the above poem, I had not read it before.