Ratites

A ratite is any of a diverse group of large, flightless birds of the order Palaeognathae. Unlike other flightless birds, the ratites have no keel on their sternum—hence the name from the Latin ratis (for raft). Without this to anchor their wing muscles, they could not fly even if they were to develop suitable wings.

Living forms of ratites

The African Ostrich is the largest living ratite. A large member of this species can be nearly 2.8 metres (9.2 feet) tall, weigh as much as 156 kilograms (340 lb), and can outrun a horse.
The Australia emu is next in height, reaching up to 1.9 metres (6.2 ft) tall and about 50 kilograms (110 lb). Like the ostrich, it is a fast-running, powerful bird of the open plains and woodlands.
Also native to Australia and the islands to the north are the three species of cassowary. Shorter than an emu, but heavier and solidly built, cassowaries prefer thickly vegetated tropical forest. They can be very dangerous when surprised or cornered because of their razor sharp talons. In New Guinea, cassowary eggs are brought back to villages and the chicks raised for eating as a much-prized delicacy, despite (or perhaps because of) the risk they pose to life and limb. They reach up to 1.7 metres (5.6 ft) tall and weigh as much as 59 kilograms (130 lb)
South America has two species of rhea, large fast-running birds of the Pampas. The larger American rhea grows to about 1.4 metres (4.6 ft) tall and usually weighs 15 to 40 kilograms (33–88 lb).
The smallest ratites are the five species of kiwi from New Zealand. Kiwi are chicken-sized, shy, and nocturnal. They nest in deep burrows and use a highly developed sense of smell to find small insects and grubs in the soil. Kiwi are notable for laying eggs that are very large in relation to their body size. A kiwi egg may equal 15 to 20 percent of the body mass of a female kiwi. The smallest species of kiwi is the Little Spotted Kiwi, at 0.9 to 1.9 kilograms (2.0–4.2 lb) and 35 to 45 centimetres (14–18 in).