Question of the week: What birds can’t fly?

There are about 40 flightless species of birds living today. There are four key differences between birds that fly and those that can't:

1. Flightless birds have more feathers.
2. The wing bones of flightless birds are proportionally smaller to their body than other birds.
3. Flightless birds have a flat breastbone or a greatly reduced keel. Flying birds have a keel on their breastbone. The keel anchors the strong muscles that move a bird's wings during flight.
4. Flightless birds developed other survival and travel methods to fit their lifestyle. For example: Ostriches have powerful legs to run or defend themselves, Penguins are excellent swimmers, and Kakapos are nocturnal parrots with camouflage plumage.

List of recent flightless birds
(Flightless bird list from Wikipedia, the free encyclopedia)

Ratites
Ostrich
Emu
Cassowaries
Kiwis
Rheas

Podicipediformes (Grebes)
Junin Flightless Grebe
Titicaca Flightless Grebe

Pelicaniformes (Pelicans, Cormorants, et al)
Flightless Cormorant

Sphenisciformes (Penguins)
Penguins

Anseriformes (Waterfowl)
Magellanic Flightless Steamer Duck
Falkland Flightless Steamer Duck
White-headed Flightless Steamer Duck
Auckland Island Teal
Campbell Island Teal

Gruiformes (Cranes, Rails)
Woodford's Rail (probably flightless)
Weka
New Caledonian Rail
Lord Howe Woodhen
Calayan Rail
New Britain Rail
Guam Rail
Roviana Rail ("flightless, or nearly so" [Taylor 1998])
Snoring Rail
Inaccessible Island Rail
Henderson Island Crake
Invisible Rail
New Guinea Flightless Rail
Takahe
Samoan Wood Rail
Makira Wood Rail
Gough Island Moorhen
Kagu
Tasmanian Native-hen

Psittaciformes (Parrots)
Kakapo