Flightlessness has evolved in several different classes of birds over time. Some of the most well-known flightless birds come from the ratite group, which includes ostriches, emus, rheas, cassowaries, and kiwis. Ratites are believed to have evolved from flying ancestors, with flightlessness arising independently multiple times. There are also some flightless waterfowl, rails, grebes, pigeons, and songbirds. Overall, there are around 60-70 species of flightless birds alive today.
What are the characteristics of flightless birds?
Flightless birds share several anatomical adaptations for a grounded lifestyle. Their wings are typically small and underdeveloped. The keel on their breastbone, where flying birds attach powerful flight muscles, is also reduced or absent. Without the hindrance of flight, flightless birds can devote resources to other traits like running speed or egg production. They tend to have sturdy legs and feet for terrestrial locomotion. Their body mass trends much heavier than flying relatives. Feathers may differ as well – flightless birds have downy plumage for insulation rather than aerodynamic flight feathers. Behaviorally, their means of escaping predators rely on hiding, aggression, or running rather than flying away.
Which ratite birds are flightless?
There are 5 main groups of flightless ratite birds alive today:
- Ostriches – The largest and fastest living birds. Native to Africa.
- Emus – The second largest living birds. Native to Australia.
- Cassowaries – Shy rainforest birds with dagger-like claws. Native to New Guinea and northeastern Australia.
- Rheas – Quick-running birds of South America.
- Kiwis – Nocturnal and practically wingless. Native to New Zealand.
All ratites lack a keel on their breastbone and have small, rounded wings. Ostriches have the most reduced wings, which are indistinguishable as forelimbs. Kiwis have the smallest wings in proportion to their body size. These vestigial wings are hidden by the kiwi’s fluffy, hair-like feathers. Ratites also share a distinctive palate structure in their skull.
Ostriches
With heights up to 9 feet (2.7 m) and weights over 300 pounds (136 kg), the ostrich is the largest living bird. Although they cannot fly, ostriches are skilled runners. Their long, strong legs can reach speeds over 40 mph (64 kph), helping them escape predators in Africa. Their wings provide balance during running. Male ostriches court females with an elaborate mating dance and defend their territory aggressively. Ostriches also have the largest eyes of any land animal. Their light, porous bones and huge eggs are adaptations to their massive body size.
Emus
Emus are Australia’s largest native birds. They stand up to 6.2 feet (1.9 m) tall and weigh around 100 pounds (45 kg). Their short, strong legs make them adept walkers and runners who can reach 31 mph (50 kph). Emus use their wings to steer while running. They live in a variety of Australian habitats and eat fruits, insects, seeds, and flowers. Female emus lay very large green eggs and raise chicks without male assistance. Emus are curious, social birds but can defend themselves with powerful kicks if threatened.
Cassowaries
Cassowaries are solitary, shy birds who live in the rainforests of New Guinea, northeastern Australia, and nearby islands. They are heavyset, flightless birds standing over 5 feet (1.5 m) tall and weighing around 130 pounds (60 kg). Cassowaries are important seed dispersers in tropical forests. Their wings have adapted into stiff wedges used for balance. Their most distinctive feature is the large, dagger-like claw on each foot that can grow over 5 inches (12 cm) long! It is used defensively and inwards on predators. Cassowaries are strong swimmers and fruit eaters as well.
Rheas
Rheas are large, fast-running birds native to South America. They stand up to 5 feet (1.5 m) tall and weigh around 55 pounds (25 kg). Of the rheas, the greater rhea is the largest and the common rhea is the most widespread. Rheas cannot fly but have powerful legs suited for running at speeds up to 40 mph (64 kph). Their wings provide stability during running. Rheas inhabit open grasslands and scrublands. They are omnivores who eat plants, seeds, fruits, insects, and small creatures. Male rheas build a nest and raise the chicks after females lay eggs.
Kiwis
Kiwis are truly unusual ratite birds restricted to New Zealand. They are the smallest ratites, standing just 20-25 inches (50-65 cm) tall and weighing 3-8 pounds (1.3-3.6 kg). They have rounded, vestigial wings hidden under shaggy brown feathers. Without flight capabilities, their bones are extremely light and brittle. Kiwis cannot run well either. They have strong legs but lack a typical bird keel in their breastbone. Their mostly nocturnal lifestyle relies on smell and touch due to their poorly developed eyesight. Kiwis probe the ground with long bills searching for worms and insects. The female lays just one enormous egg that makes up 15% of her body mass.
What waterfowl are flightless?
Several waterfowl lineages like ducks, geese, and grebes have flightless species as well. They retain most anatomical features for flying but have wings adapted for underwater swimming instead. Flightless waterfowl include:
- Falkland Steamer Ducks – Bulky diving ducks of South America. Use wings for paddling underwater.
- Auckland Islands Teal – The world’s rarest duck. Inhabits islands near New Zealand.
- Campbell Island Teal – Live on remote Campbell Island south of New Zealand.
- Hawaiian Duck – Also called the Koloa Duck, it’s the state bird of Hawaii.
- Galapagos Cormorant – The only flightless cormorant species in the world.
- Great Grebe – A large, flightless grebe found in South America.
These waterfowl gave up flight to become more efficient swimmers and divers. Their wing bones are typically shorter and heavier compared to flying relatives. They flap wings underwater to propel through the water instead of flying through air. Their feet are set further back on the body as aquatic adaptations as well.
What rail and pigeon species are flightless?
Rails are a widespread group of marsh-dwelling birds with some flightless island species. Pigeons are also widespread birds where several isolated populations lack flight abilities. Flightless rails and pigeons include:
- Aldabra Rail – Endemic to the Aldabra Atoll in the Indian Ocean.
- Inaccessible Island Rail – Only found on Inaccessible Island in the southern Atlantic Ocean.
- Henderson Island Crake – Occurs only on Henderson Island in the South Pacific.
- St Helena Crake – Went extinct in the 16th century after humans arrived at their island.
- Dodo – An extinct, giant pigeon native to Mauritius in the Indian Ocean.
- Rodrigues Solitaire – Another extinct island pigeon related to the dodo.
Free from mainland predators, these island rails and pigeons lost the ability to fly over time. Their wings became tiny and functionless. With no need for flight, their keel disappeared as well. These birds grew larger in size compared to their flying ancestors. Sadly, human activity led to the extinction of several island-endemic flightless species.
What other orders have flightless species?
Beyond ratites, waterfowl, rails, and pigeons, a few other unusual flightless birds occur as well:
- Takahe – A colorful flightless rail native to New Zealand.
- Tit-grebes – Also called titicacas. Grebe-like birds of high Andean lakes.
- Guam Rail – Extinct in the wild. Survives in captivity.
- Kagu – A rare, unique bird from New Caledonia.
- Weka – A woodhen endemic to New Zealand.
- Steamer ducks – Large, muscled ducks of South America.
These include unusual grebes, rails, and one-of-a-kind endemic species. They represent examples of flighted orders that became flightless after colonizing remote islands. Without predators or migratory needs, wings became vestigial as these species specialized for local environments over evolutionary time.
Why did flightlessness evolve in birds?
Birds have sacrificed flight numerous times to better adapt to island habitats. Flight is energetically expensive, requiring strong flight muscles, lightweight skeletons, and large wings. For island species, flight offered little advantage with a consistent food supply and lack of mainland predators or seasonal migration needs. Over time, their wings and breastbones reduced as energy diverted to other traits like egg production, running, or swimming.
The ancestor of ratites is believed to have flown to remote landmasses before losing flight. With continental separation, flightless ratite lineages then evolved independently on different islands and continents. Waterfowl like steamer ducks gave up aerial abilities for more efficient swimming. Rails and pigeons also became flightless on remote islands worldwide. Overall, flight was abandoned when costs exceeded the benefits in island environments.
Why do flightless birds remain vulnerable to extinction?
While flightlessness evolves readily in island birds, it leaves them extremely vulnerable to introduced predators and human activity. Having lost aerial escape as an option, flightless island birds lack adequate defenses when predators suddenly arrive. Humans have introduced foreign mammals like rats, pigs, dogs, and mongooses that proved disastrous to defenseless endemic species. Overhunting and habitat loss are added threats.
The dodo is a famous example of a flightless island bird driven to extinction by humans. Many other island rails, pigeons, parrots, ibis, ducks, and songbirds have also been lost. Today, over 90% of all bird extinctions come from islands. Most remaining flightless landbirds survive just on offshore predator-free islands or in protected reserves. Without conservation efforts, flightless island birds are at high risk of vanishing in the future.
What flightless landbirds still exist today?
While many flightless island species have gone extinct, several remarkable flightless birds still persist today in the wild. The five groups of ratites are the most abundant flightless landbirds with ostriches, emus, cassowaries, rheas, and kiwis spanning Africa, South America, Australia, and New Zealand. Some key flightless island species include:
- Kakapo – A giant, nocturnal parrot of New Zealand. Critically endangered.
- Takahe – A colorful rail endemic to New Zealand alpine grasslands.
- Kagu – The sole member of its family and genus. Lives in New Caledonia.
- Weka – A woodhen found only on several New Zealand islands.
- Aldabra Rail – Lives on Aldabra Atoll in the Seychelles.
These iconic birds represent unique, flightless island lineages still hanging on today. Targeted conservation efforts on offshore predator-free islands or fenced reserves seek to protect these rarest of birds into the future.
Conclusion
Flightlessness has evolved independently across numerous bird orders over time. The lack of flight provides advantages for island species while also leaving them extremely vulnerable to introduced threats. Ratites like ostriches, emus, and kiwis demonstrate large-scale flightlessness, while island rails and waterfowl represent localized isolated examples. Though many flightless island birds have gone extinct, iconic species like kiwis, takahes, and kagus persist through careful conservation. Flightless birds stand as testaments to the incredible adaptations evolution can produce in island environments over time.