Kiwi are pear-shaped, nocturnal, flightless birds found only in New Zealand. These shy birds are long-lived, possibly reaching up to 50 years old in the wild.There are five species of kiwi. The largest is the brown kiwi (Apteryx mantelli), at 20-25 inches tall and weighing 3-10 pounds. The smallest is the little spotted kiwi (Apteryx owenii), which is only 14-18 inches tall and weighs 2-4 pounds.

At about the size of a chicken, kiwi are the smallest of the ratites, a group of birds that includes ostriches, rheas, emus, and cassowaries. 

1. Kiwi dig burrows rather than building nests. The kiwi makes its home in burrows in the forest floor, which it digs out with its strong toes and claws. Once a kiwi is an adult, it typically finds a territory and spends the rest of its life there. Kiwi are protective of their territories, so they patrol their borders every night and leave droppings at the boundaries to tell other kiwi that this area is taken.

2. They're named after a sound they make. Kiwi make a shrieky half-scream, half whistle call that sounds like "kee-wee, kee-wee." This sound helps them keep track of each other at night, when they're active. Kiwi can also grunt, snort, snuffle, and hiss.

3. Kiwi have excellent hearing. The kiwi's ears are so well developed that they can easily be seen through its head feathers.

4. Kiwi can't fly. Its wings are vestigial, and so small (only about 1 inch long) that they are invisible under the kiwi's feathers. The wings have a cat-like claw at the tip, almost like bat wings, but it is nonfunctional. Like other flightless birds, kiwi lack a special ridge on the sternum called a keel. In other birds, the flight muscles are anchored to the keel.

5. They have fluffy, hair-like feathers. Kiwi plumage consists of extremely soft, long, loose feathers that feel more like the fur of a mammal than the feathers of a bird. Kiwi also have modified feathers that serve as whiskers on their faces and around the base of their beaks. They may use these bristles like cats and rats do, to feel around in the dark.

6. Kiwi don't have hollow bones. Most flying birds have hollow bones to minimize weight and make flight possible. But kiwi bones are filled with marrow, like those of mammals.

7. Kiwi have powerful legs. They lack tails but have strong, muscular legs that make up nearly a third of their body weight. They can run quickly and use their legs and claws during fights. On each foot kiwi have three forward-facing toes, one backward-facing toe, and a small spur on the back. Their fleshy foot pads allow them to walk almost silently through the forest.

8. Kiwi are the only birds with nostrils at the tip of their beaks. The kiwi bill is long, pliable, and sensitive to touch. It's the only bird in the world with external nostrils at the end of its beak. They rely on these nostrils, and their keen sense of smell, for hunting. They probe in the ground with their long bills, sniffing out invertebrates, seeds, fruit, and small animals. Studies have shown kiwi can detect worms buried several centimeters below ground.

9. Kiwi mate for life. Kiwi typically pair off for life, but "divorces" are not uncommon. Some pair bonds have been documented at 20 years. In some kiwi species, the male does most of the egg incubation, but in others both parents help out.

10. Kiwi lay the largest egg in relation to their body size of any bird. Kiwi eggs can be anywhere from 15 to 25% of the weight of the female (compare this to 2% for the ostrich). Even though they are about the same size as chickens, kiwi eggs are about six times the size of a chicken's egg. Only one egg is produced per season. Egg production is a demanding process for the female kiwi. First, she must eat three times her normal amount of food to help the developing egg grow. A few days before the egg is laid, there is little space left inside for her stomach and she is forced to fast. Gestation seems uncomfortable for kiwi mothers, and they don't move as much during this time. To relieve some of the inflammation and discomfort caused by gestating the giant egg, female kiwi will often soak their abdomens in cool puddles. 

References and Other Resources

Apteryx mantelli (Brown Kiwi), Encyclopedia of Life. Accessed February 20, 2016 at http://eol.org/pages/130161/details.Facts and Threats to Kiwi, New Zealand Department of Conservation. Accessed February 20, 2016 at http://www.doc.govt.nz/nature/native-animals/birds/birds-a-z/kiwi/facts/.

Great spotted kiwi (Apteryx haastii), Arkive. Accessed February 20, 2016 at http://www.arkive.org/great-spotted-kiwi/apteryx-haastii/.Gudipati, S. (2007). A

pteryx australis, Animal Diversity Web. Accessed February 20, 2016 at http://animaldiversity.org/accounts/Apteryx_australis/.Matherly, C. (2000).

Apteryx haastii, Animal Diversity Web. Accessed February 20, 2016 at http://animaldiversity.org/accounts/Apteryx_haastii/.Naumann, R. (1999).

Apteryx owenii, Animal Diversity Web. Accessed February 20, 2016 at http://animaldiversity.org/accounts/Apteryx_owenii/.

North Island brown kiwi (Apteryx mantelli), Arkive. Accessed February 20, 2016 at http://www.arkive.org/north-island-brown-kiwi/apteryx-mantelli/.

Rowi (Apteryx rowi), Arkive. Accessed February 20, 2016 at http://www.arkive.org/rowi/apteryx-rowi/.

Tokoeka (Apteryx australis), Arkive. Accessed February 20, 2016 at http://www.arkive.org/tokoeka/apteryx-australis/.