Vestigiality in Flightless Birds
Over time, a small number of bird species have lost their ability to fly. Some of these, such as the steamer ducks, show no appreciable changes in their flight feathers. Some, such as the Titicaca Flightless Grebe and a number of the flightless rails, have a reduced number of primaries.
The remiges of ratites are soft and downy; they lack the interlocking hooks and barbules that help to stiffen the flight feathers of other birds. In addition, the Emu's remiges are proportionately much reduced in size, while those of the cassowaries are reduced both in number and structure, consisting merely of 5–6 bare quills. Most ratites have completely lost their rectrices; only the Ostrich still has them.
Penguins have lost their differentiated flight feathers. As adults, their wings and tail are covered with the same small, stiff, slightly curved feathers as are found on the rest of their bodies.
The ground-dwelling Kakapo, which is the world's only flightless parrot, has remiges which are shorter, rounder and more symmetrically vaned than those of parrots capable of flight; these flight feathers also contain fewer interlocking barbules near their tips.
Read more about this topic: Flight Feather
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