"The Circus Animals' Desertion" is a poem about writing poetry, so what in the world are circus animals doing in it? As it turns out, there actually aren't any. Nope, those circus animals in the title are really just a metaphor for the speaker's past poetry. And they have deserted him because he has a major case of writer's block. Now that they have deserted him, he'll have to look elsewhere for some poetic inspiration.
The irony of Yeats's situation comes through pretty clearly in this title. After all, how many times have you ever heard of circus animals up and leaving? They're supposed to be highly trained. Usually the ringmaster is in control of the show, right? And if the poet is the ringmaster, he should probably have some power over his poetry.
But sadly, at the end of the day, it's still a circus. And a circus is not something people can actually control. We're guessing that's precisely the line of thought that goes into the title of this poem. Instead of positioning himself as the creator and controller of his own poetic world, Yeats positions himself as a more passive figure – the dude who's lucky enough to be in charge of the circus for a while, and then is left in the dust when the circus pulls up stakes and moves on.
Maybe it's the sense that there are absolutely no options left that compels Yeats to write this poem. It's precisely this sense of isolation (and, well, loneliness) that has allowed critics to claim that this poem represents a shift from Modernism to Postmodernism. To put it simply, Modernists may have believed that the world was a circus – but they still believed that the show itself could create magic. Postmodernism, however, starts with an empty lot and the smell of manure.