Many domesticated birds, such as the domestic chicken and domestic duck, have lost the ability to fly for extended periods, although their ancestral species, the red junglefowl and mallard, respectively, are capable of extended flight.
Flightlessness has evolved in many different birds independently. There were also other families of flightless birds, such as the now extinct Phorusrhacidae, that evolved to be powerful terrestrial predators. Taking this to a greater extreme, the terror birds (and their relatives the bathornithids), eogruids, gastornithiforms, and dromornithids (all extinct) all evolved similar body shapes – long legs, long necks and big heads – but none of them were closely related. Furthermore, they also share traits of being giant, flightless birds with vestigial wings, long legs, and long necks with some of the ratites, although they are not related.
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Divergences and losses of flight within ratite lineage occurred after the K-Pg extinction event wiped out all non-avian dinosaurs and large vertebrates 66 Ma The immediate evacuation of niches following the mass extinction provided opportunities for Palaeognathes to distribute and occupy novel environments. New ecological influences selectively pressured different taxon to converge on flightless modes of existence by altering them morphologically and behaviorally. The successful acquisition and protection of a claimed territory selected for large size and cursoriality in Tertiary ancestors of ratites Temperate rainforests dried out throughout the Miocene and transformed into semi arid deserts causing habitats to be widely spread across the growingly disparate landmasses. Cursoriality was an economic means of traveling long distances to acquire food that was usually low laying vegetation and more easily accessed by walking Traces of these events are reflected in ratite distribution throughout semi-arid grasslands and deserts today