The method; Experimental part
I developed a lightweight system consisting of a transportable minidisk recorder (various types supplied by Sony or JVC), fed by a long range microphone (Sennheiser MKH 70PH48 including an effective professional windshield) with highest sensitivity and resolution powered by an amplifier ("booster" Prefer Phantom), digital filing and two different computer based Fast Fourier Transformation analyses.
In the area where I developed the method (Hamburg), it is preferred to simply wait for the cranes to call. They do this relatively reliably in the morning up to 1 hours before sunrise, also often in the evening before sunset, and often during the day. As this area is easily accessible, I can combine a hike or other observation with recording. For this purpose, the recorder is on hold, and I react as quickly as possible as soon as I hear a crane calling.