Kakapo breed on a lek system, unique among parrots. In a breeding year males walk to traditional hilltop sites, dig bowls up to about half a metre across, connect them with cleared tracks, and boom for hours each night, sometimes for months.
The boom is produced by inflating the thorax and sits at a very low frequency, in the region of tens of hertz. Low sound travels far through vegetation without being scattered, and reports describe booms audible several kilometres away. The trade-off is directionality: low frequencies are hard to localise, which is why males follow the boom with a higher-pitched metallic ching that gives the female a bearing.
Breeding is tied to mast fruiting of trees such as rimu, so the birds do not attempt to nest every year. That, plus a small founder population and low fertility, is why total numbers are counted in the hundreds and every individual is tracked.