The loss of flight has occurred repeatedly across the globe, resulting in a diverse collection of grounded species. Currently, the ornithological consensus holds that there are approximately 60 living species of birds that have lost the ability to fly. They demonstrate how organisms can thrive by abandoning flight in favor of terrestrial or aquatic specialization.
Defining Flightlessness by Anatomical Features
Flightless birds are physically distinct from their flying relatives due to specific anatomical modifications. The most recognizable difference is the sternum, or breastbone, which lacks the large, prominent projection called the keel. This keel serves as the anchor point for the powerful pectoral muscles needed for wing movement in flying birds. Its reduction or absence in flightless species signifies a corresponding reduction in flight musculature.
The structure of the wings is also heavily altered and reduced. Instead of functioning as airfoils, the wings may be vestigial, used for balance while running, or modified into stiff, paddle-like flippers for underwater propulsion, as seen in penguins. Unlike the stiff, interlocking feathers of flying birds, many flightless species, such as kiwis and emus, have softer, looser, and more hair-like feathers that provide insulation rather than lift. Flightless birds generally possess denser, non-hollow bones, which adds weight, making flight mechanically impossible but providing strength for terrestrial locomotion.
The Global Count and Taxonomic Diversity
The 60 living species of flightless birds are scattered across several distinct taxonomic orders, showing that flightlessness has evolved independently many times. The largest and most ancient group is the Ratites, which includes the Common Ostrich, the world’s largest bird, and the Australian Emu, known for their powerful running legs. Cassowaries and the five species of Kiwi, all native to New Zealand, also belong to this group.
Another major group is the Sphenisciformes, encompassing all 18 species of Penguins, such as the Emperor Penguin and the Galapagos Penguin. These birds represent an aquatic lineage where wings have been repurposed for swimming. The remaining flightless species are scattered across various families, most notably the Rallidae, or rails. This family includes the flightless Weka and the critically endangered Kakapo, the world’s only flightless parrot. The smallest known flightless bird, the Inaccessible Island Rail, is also a member of the rail family.
Evolutionary Drivers of Flight Loss
The evolution of flightlessness is driven by a shift in the balance of costs and benefits associated with maintaining flight. Flight is metabolically expensive, requiring significant investment in large chest muscles and specialized bone structures. When flight benefits are reduced, natural selection favors individuals who reallocate this energy to other functions, such as larger body size, increased reproductive output, or stronger legs for running.
The most common scenario for flight loss is on isolated islands where terrestrial predators, particularly mammals, are naturally absent. Without the need to escape aerial threats, the selective pressure to maintain a costly flight apparatus diminishes, leading to a grounded lifestyle. This is pronounced in the rail family, which has evolved flightlessness on numerous islands. For aquatic species like penguins, optimizing movement in water is key; the streamlined body and flipper-like wings are incompatible with aerial flight.
Conservation Status and Threats
Flightless birds are disproportionately vulnerable to extinction despite their successful adaptation to specialized niches. Having evolved in environments largely free of ground-based predators, they often lack the behavioral defenses necessary to cope with new threats. This vulnerability is a major factor in their precarious conservation status.
Human colonization has frequently led to the introduction of invasive mammalian predators, such as rats, cats, and stoats, to the isolated ecosystems where flightless birds thrive. These non-native predators have decimated populations of grounded birds, which often build nests on the ground and cannot easily flee. Of the approximately 226 flightless bird species known from the fossil record and the present, 166 have already gone extinct, mostly due to human-driven causes. Today, about half of the surviving 60 flightless species are listed as threatened or vulnerable to extinction.