Two key differences between flying and flightless birds are the smaller wing bones of flightless birds and the absent (or greatly reduced)
keel on their breastbone, which anchors muscles needed for wing movement. Repeated selection for
cursorial traits across ratites suggests these adaptions comprise a more efficient use of energy in adulthood. However, ratite anatomy presents other primitive characters meant for flight, such as the fusion of wing elements, a cerebellar structure, the presence of a
pygostyle for tail feathers, and an
alula on the wing. Some flightless varieties of island birds are closely related to flying varieties, implying flight is a significant
biological cost. A study looking at the basal rates of birds found a significant correlation between low basal rate and pectoral muscle mass in kiwis. On the contrary, flightless penguins exhibit an intermediate basal rate. This is likely because penguins have well-developed pectoral muscles for hunting and diving in the water. Species with certain characteristics are more likely to evolve flightlessness. For example, species that already have shorter wings are more likely to lose flight ability. Some species will evolve flatter wings so that they move more efficiently underwater at the cost of their flight. Additionally, birds that undergo simultaneous wing molt, in which they replace all of the feathers in their wings at once during the year, are more likely to evolve flight loss. A number of bird species appear to be in the process of losing their powers of flight to various extents. These include the
Zapata rail of
Cuba, the
Okinawa rail of
Japan, and the
Laysan duck of
Hawaii. All of these birds show adaptations common to flightlessness, and evolved recently from fully flighted ancestors, but have not yet completely given up the ability to fly. They are, however, weak fliers and are incapable of traveling long distances by air.
Continued presence of wings in flightless birds Although
selection pressure for flight was largely absent, the wing structure has not been lost except in the New Zealand moas. High parental involvement denotes the necessity for choosing a reliable mate. In a climatically stable habitat providing year-round food supply, a male's claimed territory signals to females the abundance of resources readily available to her and her offspring. Penguins evolved their wing structure to become more efficient underwater at the cost of their efficiency in the air. The only known species of flightless bird in which wings completely disappeared was the gigantic, herbivorous
moa of
New Zealand, hunted to extinction by humans by the 15th century. In moa, the entire
pectoral girdle is reduced to a paired
scapulocoracoid, which is the size of a finger. ==List of flightless birds==