The kakapo is the only parrot to have a lek mating system (5); early in the breeding season (between December and April) (3), males gather on display grounds where a number of bowl shaped depressions are dug out in the ground. Having competed for access to the best locations, a male settles into a bowl and then begins to 'boom' to attract females (5). This strange, very low frequency call can be heard up to five kilometres away, and obtains its resonance via inflatable throat air sacs; lek-displaying males also make a metallic, high pitched 'ching' call (5). After mating, female kakapos incubate the eggs and rear the chicks alone. Two to three eggs are usually produced and the chicks hatch after 30 days (5). Sexual maturity is not reached until nine to ten years of age; furthermore, breeding is erratic and slow, occurring every two to five years, and is dictated by the infrequent availability of super-abundant food supplies (3). One such event is the 'mast fruiting' of the 'rimu' tree (Dacrydium cupressinum), which only occurs every two to five years (7). The kakapo feeds on a variety of fruits, seeds, roots, stems, leaves, nectar and fungi (5). Today, introduced plants are important foods on some islands (3).