Rimu, Dacrydium cupressinum, fruits irregularly. In most years the crop is negligible; every two to four years, and sometimes less often, the trees produce a heavy synchronous mast. Kākāpō breeding tracks that cycle, and in non-mast years most females do not nest at all.
The coupling is not simply an abundance of food. Managers have found that the ripeness and density of the developing rimu crop predicts breeding, and rangers survey fruit on marked trees to forecast a season months ahead. Kākāpō are lek breeders: males dig bowls, inflate a thoracic air sac and boom for weeks, and females travel to them and rear chicks alone.
Supplementary feeding can bring females into condition, but it has side effects, including skewing the sex ratio of chicks toward males in some circumstances. The constraint is therefore biological rather than purely logistical, and it is why the recovery curve is a staircase rather than a slope.