Analysis of It is not Always May

No hay pajaros en los nidos de antano.
                Spanish Proverb

The sun is bright,--the air is clear,
  The darting swallows soar and sing.
And from the stately elms I hear
  The bluebird prophesying Spring.

So blue you winding river flows,
  It seems an outlet from the sky,
Where waiting till the west-wind blows,
  The freighted clouds at anchor lie.

All things are new;--the buds, the leaves,
  That gild the elm-tree's nodding crest,
 And even the nest beneath the eaves;--
   There are no birds in last year's nest!

All things rejoice in youth and love,
   The fulness of their first delight!
 And learn from the soft heavens above
   The melting tenderness of night.

Maiden, that read'st this simple rhyme,
   Enjoy thy youth, it will not stay;
Enjoy the fragrance of thy prime,
   For oh, it is not always May!

Enjoy the Spring of Love and Youth,
   To some good angel leave the rest;
For Time will teach thee soon the truth,
  There are no birds in last year's nest!