Many years ago, I was involved in a project mapping the nesting sites in Ireland of a bird called the "corncrake", Its Latin name is Crex crex - which is a reasonable approximation of the grating noise which is the nearest it gets to singing! Anyway, one way of locating birds was to get two pieces of a beef rib, cut a sawtooth along the edge of one and use the other as a rasp. With practice, you could get the pattern and duration right to roughly imitate the call. All that was then necessary was to tune the sound by shaping your hand into a cup shape and varying it until you got the sound right! Any male corncrake in the vicinity would immediately take offence and rasp back at you!

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