| Curator Brad Livezey
Evolution of Avian Flightlessness Species of birds in a number of taxonomic families are permanently flightless. Modern examples evidently descended from fully flighted relatives, and most (but not all) occur on oceanic islands. In addition to the comparatively well known ratites (ostriches and allies) and penguins, scattered throughout the globe are (were) flightless ducks, cormorants, grebes. rails, pigeons, and parrots, among others See: Steamer Ducks (Tachyeres) My research into the evolutionary loss of flight in birds began as a doctoral student at the University of Kansas. The subject of that effort were the steamer-ducks (Anatidae: Tachyeres) of southern South America, a genus comprising three flightless and one flighted species. The three flightless forms occur in coastal habitats and occupy allopatric ranges, whereas the flighted form occurs throughout the ranges of the former three (whrere some heavy individuals also are flightless) as well as on freshwater Andean lakes. All members of the genus are similar in appearance, being bulky, gray-and-white ducks with orangish bills and feet. The flightless species differ most in the ratio of body mass to wing area, a parameter critical to flight but which provides few clues to identity in the field. As a result, the taxonomy of the group was uncertain for more than a century following their original discovery. Charles Darwin was one of the first to describe the peculiar, turblent method of surface locomotion characteristic of the genus, and this behavior formed the basis for the common name likening the birds to contemporary paddle-wheel steamers. Put simply, the flightless members of the genus simply underwent an evolutionary increase in mass that was disproportionate to wing size, and all evidence indicates that members of the flightless species are unable to attain flight at any time during their development or lifetimes. Steamer-ducks are very unusual in inhabitation of continental shorelines, their size, pugnacity, and various aquatic means of escape permitting them to survive under non-insular conditions See:
Other Flightless Aquatic Birds Other flightless waterfowl are insular, and include the Auckland Islands Flightless Teal, several extinct geese in Hawaii, and an extinct sea-duck from coastal California. The Galapagos Cormorant ("Nannopterum" harrisi) is a flightless diving bird endemic to two islands in the archipelago, and three flightless grebes are scattered on single freshwater lakes in the Neotropics, exemplifying a different class of insularity through endemism of aqautic birds on continental lakes. See:
Flightless Terrestrial Birds Excluding the ostriches and related cursorial ratites of southern continents, flightless land birds are strictly insular in distribution. The extinct Dodo (Raphus cucullatus), formerly of Mauritius in the Indian Ocean, was a large, flightless pigeon. A close relative from the island of Rodriguez, the Solitaire (Pezophaps solitaria), also was a giant, flightless pigeon; unlike the Dodo, however, the Solitaire evolved bony wing knobs used in territorial encounters. Both species, especially the Dodo, evidently assumed giant stature and lost the capacity for flight through a combination of developmental mechanisms, paedomorphosis of the pectoral limb and peramorphosis of the rest of the body. Comparatively poorly known among flightless birds is the critically endangered parrot or Kakapo (Strigops habroptilus) of New Zealand. The subalpine forest bird travels on foot and subsists on vegatation digested in its enormous crop, the latter occupying much of the volume of the breast normally enclosing the breast muscles in flighted parrots. The family of birds most inclined to undergo evolutionary loss of flight, the rails (Rallidae) include at least fifty extant and historically extinct flightless members. Flightless rails span more than two orders of magnitude in body mass, and include examples of dwarfism, stasis, and giantism among flightless members. All flightless rails are (were) endemic to islands (including New Zealand), and include a diversity of life histories. Most or all examples manifest signs of paedomorphosis (juvenile traits retained into adulthood) of the pectoral limb. Unfortunately, vulnerability to rats and other introduced predators have driven many flightless rails to extinction in historical times. See:
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Introduction