The quick answer to this question is yes, a flightless bird is still considered a bird even if it cannot fly. The ability to fly is not a requirement for an animal to be classified as a bird. As long as the animal shares other key characteristics with birds, such as feathers, wings, laying hard-shelled eggs, and a beak, flightlessness does not disqualify it from being a member of class Aves.
What makes something a bird?
Biologists classify animals into different groups based on shared physical and genetic characteristics. The group of air-breathing vertebrate animals known as “birds” is classified as the class Aves. While there is some debate among scientists about the exact parameters for class Aves, most agree that the following characteristics are required for an animal to be considered a true bird:
- Endothermic (warm-blooded)
- Lay hard-shelled eggs
- Have feathers
- Have wings
- Have a beak with no teeth
- Have a unique bone structure including a furcula (wishbone)
- Have high metabolism
The ability to fly is not considered a requirement. Birds evolved from feathered dinosaurs over 150 million years ago during the Jurassic Period. Not all of those ancestral birds could fly. The earliest birds such as Archaeopteryx likely spent much of their time on the ground and trees rather than flying. Over time, some lineages of birds evolved adaptations for flight while others did not.
Why did some bird lineages lose the ability to fly?
Flight provides many advantages for birds – the ability to escape predators, access new habitat and food resources, migrate long distances, and more. However, developing and maintaining the ability to fly is metabolically expensive. The various adaptations required for flight (lightweight bones, large flight muscles, wings with feathers, etc.) take energy to build and maintain.
In some ecological niches, the benefits of flight are outweighed by the costs. On remote islands that lack ground predators, for example, the ability to fly offers little advantage. Over many generations, birds in these environments may lose their ancestral ability to fly through natural selection. Any mutations that reduce the size of wings or flight muscles would conserve energy. In these situations, being flightless becomes advantageous, so those traits spread through the population. The same process has led to loss of flight in some bird lineages that specialized in diving for food rather than flying such as penguins.
Example flightless birds
There are around 40 species of flightless birds alive today, all belonging to orders that have other flying members. Some examples include:
- Ostriches – The largest living birds, native to Africa. They can grow over 9 feet tall and weigh over 300 pounds.
- Cassowaries – Large, flightless birds found in the forests of New Guinea and northeastern Australia.
- Kiwis – Small brown birds with a long beak native to New Zealand. They are the smallest living ratite birds.
- Penguins – Flightless seabirds found throughout the southern hemisphere. They use their wings for swimming instead of flying.
- Rheas – Large, flightless birds native to South America. Related to ostriches and emus.
There are also many species of flightless island rails throughout the Pacific and Indian Oceans. These birds evolved from flying ancestors after reaching remote islands without ground predators.
Unique adaptations of flightless birds
Becoming flightless allowed birds to make dramatic alterations to their anatomy that would have negatively impacted flight. Without the constraint of needing wings, some lineages evolved to become giant terrestrial browsers. The tallest and heaviest birds that ever lived were flightless – such as the 9 foot tall Dromornis stirtoni of Australia that weighed over 1100 pounds and the 10 foot tall Vorombe titan of Madagascar that weighed over 1600 pounds.
Other flightless birds show more modest anatomical changes compared to their flying relatives. Most species have smaller wings relative to their body size. Ostriches, cassowaries, kiwis and allies also have primitive feathers without interlocking barbules. In flying birds, these interlocking barbules help the feather resist air pressure during flight. Without this need, the feathers became simpler over time.
While reduced wings and feathers are a common adaptation, different flightless lineages show unique changes as well. Penguins, for example, evolved a streamlined body and flippers adapted for underwater swimming. Kiwis have an unusually long, thin beak specialized for probing the ground. Flighted birds tend to have much smaller pelvic and tail bones compared to flightless birds. Large pelvic bones anchor their powerful leg muscles used for running and defense. Long tail feathers aid in balance and steering while running.
Flightless bird reproduction
One major difference between flying and flightless birds is their mode of reproduction. Flying birds have relatively small yolky eggs compared to flightless birds. The parent birds must incubate these small eggs frequently to supply warmth. Flightless bird eggs contain much more yolk, allowing the embryo to develop with less frequent incubation. The parents do not need to sit on the nest constantly, freeing them to forage for food.
Flightless bird eggs reflect their greater precocial development strategy compared to flying birds. Their hatchlings can walk, feed themselves, and survive with less parental care. However, the larger eggs make the eggs and nest more conspicuous to predators. The parents must rely more on camouflage or actively defending their nest rather than hiding it.
Egg size comparison
| Bird type | Example species | Egg width | Incubation strategy |
|---|---|---|---|
| Flying bird | Rock pigeon | 1.5 inches | Frequent incubation needed |
| Flightless bird | Ostrich | 6 inches | Less frequent incubation |
As this table shows, the ostrich egg is over 4 times wider than a pigeon egg, reflecting the increased yolk and decreased need for constant incubation.
Relationship to flying relatives
All living flightless birds belong to avian groups that have other members capable of flight. Ostriches are part of order Struthioniformes, which includes other flying birds like rheas, emus, and kiwis. Their closest living relative is the Somali ostrich, which can fly to some degree. Penguins belong to order Sphenisciformes, which has one flying species – the Galapagos penguin. Most other penguins areflightless.
This pattern indicates flightless birds evolved from flying ancestors within these orders. They did not diverge before flight evolved. Their unique adaptations are the result of evolutionary changes over time after a flying population reached an environment favoring flightlessness. They represent one evolutionary path among modern bird diversity.
Flightless bird evolution
| Age | Event |
|---|---|
| Late Jurassic | Birds evolve flight from therapod dinosaurs |
| Cretaceous | Ancestral populations of flightless bird orders reach remote islands |
| Cenozoic | Flightlessness evolves independently in various insular and aquatic lineages |
| Present | ~40 living flightless bird species remain worldwide |
This table summarizes the evolutionary history that produced modern flightless birds from flying ancestors over the past 150+ million years.
Conclusion
In summary, flightless birds meet the requirements to be classified as members of class Aves, even if they cannot fly themselves. Lack of flight represents an evolutionary adaptation to certain ecological niches, but does not disqualify an animal from being considered a bird. All living flightless birds belong to lineages closely related to flying birds. Their unique traits related to flightlessness, such as small wings and large eggs, reflect convergent evolution in these isolated populations. While losing the ability to fly may seem disadvantageous, it has allowed some remarkable birds to thrive in habitats where flying offered little benefit. Their existence provides insight into the diverse evolutionary paths among modern bird diversity.
