Milegajo, interesting post but why not move into the twenty first century. What I mean by this is do it digitally there some great bird call recognition sites ie RSPB and others, make some recordings of your target species and then play them back through a music playing device with loud speakers. My wife has had great success doing this from her I phone and her I pad, she,s into bird photography . You could have crows gulls pigeon rabbit hare fox and deer all on the one device. Cheers Sherlock.