The point of the "echo" reference is probably that the rasp of the corncrake tends to bounce off the gable end of cottages and head back towards the bird - who then ends up responding to this "rival"! At one stage, many years ago, I used to survey these birds in the Irish Midlands. We used a crude directional microphone (actually part of a child's "spy kit" received as a present by somone's children) to locate the birds and separate out the echoes. We also, incidentally, raised the birds by cutting a sawtooth edge on a length of beef rib bone, using a second piece as a rasp. Very effective - and less weather-prone than a tape recorder!