A particular sound a particular male parrot makes to attract a mate.
The bird is the Kakapo, the Night Parrot of New Zealand. Its mating routines are ridiculously complicated, involving the intersections of fruit, incredibly high places and shallow depressions in the ground.
The upshot of this is that under ideal circumstances (ie, if the species were not horribly, horribly endangered) male Kakapo call for a mate every December. There are two particular sounds that are unique, as far as I can tell in both sound and terminology, to this species of parrot: booming and chinging. Booming is a low throb that gets females' attention - it carries for incredible distances but is extremely hard to trace to its source. Chinging is of a much higher frequency and a much shorter range and functions as a homing beacon.