I think that this is about someone contemplating suicide because he believes that no one notices him and he is alone
"as I dive into the estuary, the birds do not flinch in my fall, they perch in the rafters, before, during, after. They do not notice me at all"
Birds being the people around him. He doesn't believe that anyone would notice or care if he dies
"a splash all I left in the world"
I think that this is about someone contemplating suicide because he believes that no one notices him and he is alone "as I dive into the estuary, the birds do not flinch in my fall, they perch in the rafters, before, during, after. They do not notice me at all" Birds being the people around him. He doesn't believe that anyone would notice or care if he dies "a splash all I left in the world"