information:extraction, structural:expansion, quality:corruption, structural:summarization
Flight and flightlessness

The wings of all rails are short and rounded. The flight of those Rallidae able to fly, while not powerful, can be sustained for long periods of time, and many species migrate annually. The weakness of their flight, however, means they are easily blown off course, thus making them common vagrants, a characteristic that has led them to colonize many isolated oceanic islands. Furthermore, these birds often prefer to run rather than fly, especially in dense habitat. Some are also flightless at some time during their moult periods.

Flightlessness in rails is one of the best examples of parallel evolution in the animal kingdom. Of the roughly 150 historically known rail species, 31 extant or recently extinct species evolved flightlessness from volant (flying) ancestors. This process created the endemic populations of flightless rails seen on Pacific islands today.

Many island rails are flightless because small island habitats without mammalian predators eliminate the need to fly or move long distances. Flight makes intense demands, with the keel and flight muscles taking up to 40% of a bird's weight. Reducing the flight muscles, with a corresponding lowering of metabolic demands, reduces the flightless rail's energy expenditures. For this reason, flightlessness makes it easier to survive and colonize an island where resources may be limited. This also allows for the evolution of multiple sizes of flightless rails on the same island as the birds diversify to fill niches.

In addition to energy conservation, certain morphological traits also affect rail evolution. Rails have relatively small flight muscles and wings to begin with. In rails, the flight muscles make up only 12–17% of their overall body mass. This, in combination with their terrestrial habits and behavioral flightlessness, is a significant contributor to the rail's remarkably fast loss of flight; as few as 125,000 years were needed for the Laysan rail to lose the power of flight and evolve the reduced, stubby wings only useful to keep balance when running quickly. Indeed, some argue that measuring the evolution of flightlessness in rails in generations rather than millennia might be possible.

Another factor that contributes to the occurrence of the flightless state is a climate that does not necessitate seasonal long-distance migration; this is evidenced by the tendency to evolve flightlessness at a much greater occurrence in tropical islands than in temperate or polar islands.

It is paradoxical, since rails appear loath to fly, that the evolution of flightless rails would necessitate high dispersal to isolated islands. Nonetheless, three species of small-massed rails, Gallirallus philippensis, Porphyrio porphyrio, and Porzana tabuensis, exhibit a persistently high ability to disperse long distances among tropic Pacific islands, though only the latter two gave rise to flightless endemic species throughout the Pacific Basin. In examining the phylogeny of G. philippensis, although the species is clearly polyphyletic (it has more than one ancestral species), it is not the ancestor of most of its flightless descendants, revealing that the flightless condition evolved in rails before speciation was complete.

A consequence of lowered energy expenditure in flightless island rails has also been associated with evolution of their "tolerance" and "approachability". For example, the (non-Rallidae) Corsican blue tits exhibit lower aggression and reduced territorial defense behaviors than do their mainland European counterparts, but this tolerance may be limited to close relatives. The resulting kin-selecting altruistic phenomena reallocate resources to produce fewer young that are more competitive and would benefit the population as an entirety, rather than many young that would exhibit less fitness. Unfortunately, with the human occupation of most islands in the past 5,000 to 35,000 years, selection has undoubtedly reversed the tolerance into a wariness of humans and predators, causing species unequipped for the change to become susceptible to extinction.