The Kakapo is the only species of flightless parrot in the world, and the only flightless bird that has a lek breeding system. Males loosely gather in an arena and compete with each other to attract females. Females listen to the males as they display, or "lek". They choose a mate based on the quality of his display; they are not pursued by the males in any overt way. No pair bond is formed; males and females meet only to mate.
During the courting season, males leave their home ranges for hilltops and ridges where they establish their own mating courts. These leks can be up to 7 kilometres (4 mi) from a Kakapo's usual territory and are an average of 50 metres (160 ft) apart within the lek arena. Males remain in the region of their court throughout the courting season. At the start of the breeding season, males will fight to try to secure the best courts. They confront each other with raised feathers, spread wings, open beaks, raised claws and loud screeching and growling. Fighting may leave birds with injuries or even kill them.
Each court consists of one or more saucer-shaped depressions or "bowls" dug in the ground by the male, up to 10 centimetres (4 in) deep and long enough to fit the half-metre length of the bird. The Kakapo is one of only a handful of birds in the world which actually constructs its leks. Bowls are often created next to rock faces, banks, or tree trunks to help reflect sound - the bowls themselves function as amplifiers to enhance the projection of the males booming mating calls. Each male’s bowls are connected by a network of trails or tracks which may extend 50 metres (160 ft) along a ridge or 20 metres (60 ft) in diameter around a hilltop. Males meticulously clear their bowls and tracks of debris. One way researchers check whether bowls are visited at night is to place a few twigs in the bowl; if the male visits overnight, he will pick them up in his beak and toss them away.
To attract females, males make loud, low-frequency (below 100 Hz) booming calls from their bowls by inflating a thoracic sac. They start with low grunts, which increase in volume as the sac inflates. After a sequence of about 20 loud booms, the male Kakapo emits a high frequency, metallic "ching" sound. He stands for a short while before again lowering his head, inflating his chest and starting another sequence of booms. The booms can be heard at least one kilometre (0.6 mi) away on a still night; wind can carry the sound at least five kilometres (3 mi). Males boom for an average of eight hours a night; each male may produce thousands of booms in this time. This may continue every night for three or four months during which time the male may lose half his body weight. Each male moves around the bowls in his court so that the booms are sent out in different directions. These booms are also notorious for attracting predators, because of the long range at which they can be heard.
Females are attracted by the booms of the competing males; they too may need to walk several kilometres from their territories to the arena. Once a female enters the court of one of the males, the male performs a display in which he rocks from side to side and makes clicking noises with his beak. He turns his back to the female, spreads his wings in display and walks backwards towards her. The duration of attempted copulation is between 2 to 14 minutes. Once the birds have mated, the female returns to her home territory to lay eggs and raise the chicks. The male continues booming in the hope of attracting another female.
The female Kakapo lays up to three eggs per breeding cycle. She nests on the ground under the cover of plants or in cavities such as hollow tree trunks. The female incubates the eggs faithfully, but is forced to leave them every night in search of food. Predators are known to eat the eggs and the embryos inside can also die of cold in the mother's absence. Kakapo eggs usually hatch within 30 days, bearing fluffy gray chicks that are quite helpless. After the eggs hatch, the female feeds the chicks for three months, and the chicks continue to remain with the female for some months after fledging. The young chicks are just as vulnerable to predators as the eggs, and young have been killed by many of the same predators that attack adults. Chicks leave the nest at approximately 10 to 12 weeks of age. As they gain greater independence, their mothers may feed the chicks sporadically for up to 6 months.
Because the Kakapo is long-lived, with an average life expectancy of 95 years and the maximum at about 120 years, it tends to have an adolescence before it starts breeding. Males do not start to boom until about 5 years of age. It was thought that females reach sexual maturity at 9 years of age; but this idea was debunked in the 2008 breeding season when two 6-year-old females named Apirama and Rakiura laid eggs. Generally females do not seek out males until they are between 9 and 11-years-old. The Kakapo does not breed every year and has one of the lowest rates of reproduction among birds. Breeding occurs only in years when trees mast (fruit heavily), providing a plentiful food supply. Rimu mast occurs only every three to five years, so in rimu-dominant forests such as those on Codfish Island, Kakapo breeding occurs as infrequently.
Another interesting aspect of the Kakapo's breeding system is that a female can alter the sex ratio of her offspring depending on her condition. A female that eats protein-rich foods produces more male offspring (males have 3%–40% more body weight than females). Females produce offspring biased toward the dispersive sex when competition for resources (such as food) is high and toward the non-dispersive sex when food is plentiful. A female Kakapo will likely be able to produce eggs even when there are few resources, while a male Kakapo will be more capable of perpetuating the species when there are plenty, by mating with several females. This supports the Trivers–Willard hypothesis. The relationship between clutch sex ratio and maternal diet has conservation implications, because a captive population maintained on a high quality diet will produce fewer females and therefore fewer individuals valuable to the recovery of the species.
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