Small greenish birds are working through the willows and the garden shrubs right now, flicking branch to branch and almost never holding still. These are warblers heading south, and most of them fly at night, which is why a thicket that was empty yesterday can be busy at first light. There's no song to go by in autumn, and the birds have traded their spring colours for a dull yellow green that even experienced birders give up on naming. That is the freeing part, because you don't have to know which one it is to know that migration is happening in your hedge. By early October the shrubs go quiet again.