Begin by listening to see if you can differentiate the two cranes in each unison call. Once you realize there are two birds calling, describe how the one bird calls and what the other bird calls in between. Listen to the calls of TWO crane species. Listen to compare and hear how they are different. You'll hear more each time you listen.
Most people never get to hear whooping cranes call.
If you visit the refuge, they don't call in the daytime, or it's so far away you can't distinguish male and female. Even people who have heard the call have not distinguished male and female. You can definitely hear it. Listen over and over again until you can say, 'I have differentiated two voices in the unison call. Most listeners think the call of the Eurasian crane sounds clearer, more like a pure musical instrument.