The only sure way to differentiate crane sexes, is their calling behavior. Males, the crane on the left in the photo below, extend their heads near vertical and “karoo” loudly while females move their heads up and down to 45 degrees and make two quick cluck-like calls for every call made by the male. This synchronized duet is thought to be how a mated pair reinforces their life-long bond and also to threaten predators or intruding cranes.