Kiwi partnerships have been known to last longer than 20 years. Every third day or so the pair comes together to share a burrow, and at night they perform duets, calling to each other.
Divorces also happen, especially in high-kiwi density areas in Northland. Divorces can be caused by breeding failures, or if birds are young and early in their breeding career.
The main breeding season runs from June to March, when food is most plentiful. The exception is brown kiwi in the North Island, which can lay eggs in any month.
In captivity, male kiwi can reach sexual maturity at 18 months, and females can lay their first eggs when about three years old. In the wild, kiwi usually do not breed until much older. Wild females lay their first eggs at 3-5 years of age.
In rowi, Operation Nest Egg has changed some breeding behaviour—birds which are re-released into their wild home tend to breed at an earlier age than those raised by their parents. It is thought this is because wild-raised rowi live in family groups, with young birds staying in their parents’ territory for several years to help raise their siblings. Operation Nest Egg breaks these family ties, which means young birds released back into the sanctuary are free to mate sooner. The positive result is that Operation Nest Egg birds are helping boost rowi’s small population more quickly.
With no colourful plumage or a beautiful song to attract his mate, the male kiwi has developed the strategy of persistence. He follows her about, grunting. If uninterested, she may run away, or use her greater weight and size to see him off. However, if she is interested, mating takes place, three or more times a night during the peak of activity.
The male taps or strokes the female on her back, near the base of her neck. She crouches low with her head stretched forward and resting on the ground. Because the female is the larger bird, the male needs her full co-operation. He climbs onto her back, which can be difficult with no wings or tail to help him balance. Often he will grasp her back feathers in his beak to help his balance.
The kiwi female calls the shots during mating. If she loses interest she may wander away, leaving the male in an undignified heap on the ground.
Two Functional Ovaries
Another unusual thing about kiwi is that females have two functional ovaries. In most other birds only the left ovary develops. Only brown kiwi regularly lay more than one egg in a clutch, but when this occurs, ovulation happens in each ovary.