How Big Is a Kiwi Egg? Huge Eggs for a Chicken-Sized Bird

How big is a kiwi egg compared with the bird?

The North Island brown kiwi is about 40 cm (16 in) long, and females typically weigh 2.1–3.9 kg (4.5–8.5 lb). Yet each egg weighs around 430 g (15 oz) on average, with large eggs reaching about 450–470 g (1 lb). For a typical female of about 2.7 kg (6 lb), that is roughly 15–20 percent of her body weight.

Across the five kiwi species, eggs usually weigh about 15–25 percent of the female’s weight. The highest ratios belong to the smallest kiwi. The little spotted kiwi, at only about 1–2 kg (2.2–4.4 lb), lays eggs averaging about 300 g (11 oz), close to a quarter of its body weight.

The egg is smooth, ivory or pale greenish white and long and oval. A brown kiwi egg is about 12–13 cm (4.7–5.1 in) long, roughly the length of a smartphone.

Kiwi egg vs chicken egg and ostrich egg

Comparing eggs by absolute size and by size relative to the mother gives very different winners.

The ostrich lays the biggest egg of any living bird, but compared with its own body it is one of the smallest (see How big is an ostrich egg?). The kiwi is the reverse: a small bird with one of the largest eggs for its size of any bird. If a human mother gave birth to a baby of the same relative size, it would weigh around 12–15 kg (26–33 lb).

What is inside a kiwi egg?

Most of a kiwi egg is yolk. The yolk makes up about 65 percent of the contents, compared with roughly 35–40 percent in the eggs of most birds. This rich food store lets the embryo develop for a long time inside the shell.

Incubation is slow: about 63–92 days depending on the species, and usually about 75–90 days in the North Island brown kiwi. That is several times longer than the 21 days of a chicken egg. Hatching itself is hard work, and a chick may take two or three days to break out.

The chick emerges fully feathered, looking like a miniature adult. It still carries a large part of the yolk inside its belly, which feeds it for its first days of life, sometimes up to about two and a half weeks. Kiwi parents do not feed their chicks, so the chick leaves the nest to find its own food after about a week.

How does a female kiwi carry such a big egg?

Producing the egg puts a heavy strain on the female. It takes about a month to grow a fully developed egg, and during that time she eats around three times her normal amount of food. In the last two or three days before laying, the egg fills so much of her body that there is little room left for her stomach, and she stops eating.

Kiwi are also unusual inside. Unlike most birds, which have only one working ovary, female kiwi have two functioning ovaries, though eggs pass through a single oviduct. Flightlessness helps too, since a flying bird could not carry such a heavy load.

North Island brown kiwi can lay in any month, with a peak from June to November. A clutch usually has one or two eggs laid about three weeks apart, and a pair may raise two clutches in a year. The great spotted kiwi, in contrast, usually manages only one egg a year.

Who incubates the kiwi egg?

In the North Island brown kiwi, only the male incubates. He sits on the egg in a burrow, hollow log or dense vegetation for most of each day and night, leaving only briefly to feed, and he can lose a noticeable share of his body weight over the long incubation. In some other kiwi, such as the great spotted kiwi, both parents take turns on the nest.

Kiwi are not the only flightless birds with devoted fathers. Male emus also incubate alone and barely eat for about eight weeks, as explained in Do male emus sit on the eggs?

Why do kiwi lay such big eggs?

For many years, scientists suggested that the kiwi descended from a much larger, moa-like ancestor. In this view, the big egg was a leftover from a time when kiwi ancestors were bigger, and the birds shrank while the egg stayed large.

DNA studies have challenged that idea. The kiwi’s closest known relatives are the extinct elephant birds of Madagascar, not the moa, and a small fossil kiwi relative from New Zealand, Proapteryx, was probably able to fly. This suggests the kiwi’s ancestors were small flying birds that reached New Zealand and later lost flight.

Today the large egg is generally seen as an adaptation in its own right. A big, yolk-rich egg produces a well-developed chick that can move and feed itself almost at once. In New Zealand before people arrived, there were no egg-eating land mammals, so a single large egg in a burrow was relatively safe.

Big eggs and kiwi survival today

That ancient strategy works poorly against introduced predators. Stoats, cats and other mammals now kill most kiwi chicks in unmanaged areas, and only around 5–10 percent survive to adulthood without protection. Because each female lays so few eggs, populations recover slowly.

Conservation programs such as Operation Nest Egg collect kiwi eggs from the wild, hatch them in incubators and return the young birds once they are big enough to defend themselves. Read more in Are kiwi birds endangered? and see other birds that cannot fly in our list of flightless birds.