The poem
Love and harmony combine, And round our souls entwine While thy branches mix with mine, And our roots together join.
Joys upon our branches sit, Chirping loud and singing sweet; Like gentle streams beneath our feet Innocence and virtue meet.
Thou the golden fruit dost bear, I am clad in flowers fair; Thy sweet boughs perfume the air, And the turtle buildeth there.
There she sits and feeds her young, Sweet I hear her mournful song; And thy lovely leaves among, There is love, I hear his tongue.
There his charming nest doth lay, There he sleeps the night away; There he sports along the day, And doth among our branches play.
This is written in the style of classic English poetry. It's sweet, almost newlywed language, evoke the "Home Sweet Home" sentiment. He(father/husband bird) is lovingly watching his mate prepare for the new birth event, and harmony resides with all three of them. The child (bird) will "sleep the night away, sport(s) along the day," and play among the branches. I like to imagine this as an extension of Blake's poem The Birds.
While I agree that Blake uses "almost newlywed language," I don't agree with the first commenter's interpretation that this poem is about a "birth event."
The first stanza seems deeply, mystically sexual. The "turtle making her nest" is almost certainly a Biblical reference, and in my humble opinion, symbolizes the fertility (in every sense) of (new) love. What I love about this poem is how simple and naive it sounds, but yet if you pause to examine it, it reveals some surprising truths!