This assumption insists on the presence of the absence of the bird.
The poem describes the memory of a bird buried in the backyard. The narrator goes to dig the bird up (why?) and finds that it’s body has disappeared.
nothing there/not even a trace/to suggest its passing/
There is no physical trace of the body but there is trace in language — the Derridean idea of trace is that every element of a system is present in every other element by the very nature of its absence. For example, the letter B contains all the letters of the alphabet because the letter B is not those other letters. This piece tells a story of the of the bird’s existence in in trace.
This assumption insists on the presence of the absence of the bird. Without the story of the missing bird, it never existed.