Absence
The title itself is suggestive of the overarching theme of the poem. The central character is gripped by the overwhelming pain that comes with the absence of a loved one from their life. The details of what caused this loss is not made clear. It may have resulted from death. It may have resulted from a break-up of a romantic relationship. Ultimately, it doesn't matter. The narrative speaks to the consequences of the absence from one's life of any person. What caused that absence is of little significance. Absence can mean literal death or merely figurative. Emotionally, there is no real difference.
Life Goes On
The protagonist is emotionally devastated by the absence of the other person. This person has returned to the last place they met to discover that it has not changed at all. The sounds of the birds have not changed, either. Despite this reality, now those same sounds have become a song mocking the sense of isolation and loss the person is now feeling. The poem is expressing the painful theme that no matter how much one's own personal life has been devastated, life goes on all around them as if nothing at all has changed.
The Paradox of Memory
The speaker asserts that the birds which are now mocking once sang a song of ecstasy. That emotion can no longer be shared. The implication is that the site at which they last met was once a happy place. Thus should mean that the place engenders happy memories. The absence of the other person now transforms those happy memories into sadness. The theme being explored here is the paradox of memory. Happy times once remembered fondly can bring on a sense of melancholy when recalled after the loss of a person who once made those memories so happy to recall.
