Ratite

The ratites are a group of flightless birds, most of which are now extinct. A number of features set them apart from other birds and provide support for their common origin. These include the absence of a keel on their breastbone, which in flying birds provides a strong anchor for their wing muscles, so ratites could not fly even if they developed suitable wings.

There are four extant groups of ratites. These may be treated as separate orders, in two orders, or as a single order Struthioniformes, as is followed here. The main groups then become suborders:

• Suborder Struthiones
  • Family Struthionidae - ostriches
• Suborder Rheae
  • Family Rheidae - rheass
• Suborder Casuarii
  • Family Casuariidae - cassowaries
  • Family Dromaiidae - emu
• Suborder Apteryges
  • Family Apterygidae - kiwis

Extinct groups include the Dinornithidae or moas. The tinamous are also sometimes included here.

Ratites are found, or were found until fairly recently, in most parts of the former supercontinent Gondwana. It has been traditionally supposed that they evolved there during the Cretaceous period, and diverged from one another as the continents separated. However, DNA analysis suggests they diverged from one another more recently, and indicates that kiwis are closer to cassowaries than to recently extinct moas, though both kiwis and moas came from New Zealand and cassowaries from Australia and the islands to the north.

Most ratites are large birds. The largest bird known was Aepyornis, the 'elephant bird' of Madagascar, which could weigh up to 450 kilograms. These went extinct some time after humans arrived on the island, possibly surviving until the late 1600s. The African ostrich is the largest bird alive today, reaching up to 3 metres tall and weighing up to 135 kilograms. The smallest ratites are the kiwis, which are the size of chickens, shy, and almost blind.