Flightless birds are a fascinating group of avian species that have lost the ability to fly over the course of evolution. Some well-known examples of flightless birds include ostriches, emus, cassowaries, kiwis, and penguins. While these birds no longer take to the skies, they retain some anatomical features related to flight, which raises an interesting question – do flightless birds have wings?
What are wings?
In birds, wings are the forelimbs that have evolved for powered flight. The wing structure contains flight feathers attached to the manus (the hand/wrist) and arm bones, which attach to the bird’s shoulder. The flight feathers are asymmetrically shaped to provide lift and thrust when flapped. The wing bones are light, yet strong, to minimize weight while retaining power. The muscles that control wing movement attach along the breast and make up a large portion of a flying bird’s body mass. In addition, flying birds have a keeled breastbone (sternum) which anchors these flight muscles. So in summary, the anatomy of a wing allows a bird to generate lift and propulsion to become airborne.
Skeletal anatomy of wings in flightless birds
Flightless birds have the same skeletal wing anatomy as flying birds, with a few key differences. The bones are not as pneumatized or hollow to decrease weight. The keel of the breastbone is small or even absent. Overall, the wings bones themselves are much reduced in size proportional to the body. For example, the Giant Moa, an extinct flightless bird from New Zealand, had wing bones estimated to be only about 20 cm long, far too small to support flight in an ostrich-sized bird. However, the basic components of the forelimb – humerus, radius, ulna, wrist, and fingers – are all still present. Vestigial flight feathers may remain attached to the wrist and fingers.
Ostrich wing anatomy
Ostriches have tiny wings relative to their huge bodies. The wingspan averages about 1.5-2 meters, compared to their 2.5-3 meter height. Their wings lack large flight feathers, having only smaller plumes and downy feathers. However, their wings still retain a basic skeletal structure consisting of a humerus, radius, ulna, carpals (wrist), metacarpals, and phalanges (fingers). Their breastbone lacks a keel and their wing muscles are proportionally small and undeveloped.
Penguin wing anatomy
Penguins have flipper-like forelimbs that function as wings underwater while swimming. Their wingspan is about 30% of their height. The wing bones are flattened and stiff to provide propulsion in water. The bones are solid rather than pneumatized. The shoulder bones are large where swimming muscles attach. The breastbone may have a small vestigial keel. Penguins have scale-like feathers on the surface of their stiff wing bones that smooth the wings for swimming.
Muscular anatomy of wings in flightless birds
Flightless birds lack the large, powerful flight muscles of birds that can fly. Their smaller pectoral muscles may aid inDisplayNamewing folding or other limb movements, but are not capable of powering flight. For example, the large pectoralis muscles that make up 15-25% of a flying bird’s body mass are almost non-existent or very reduced in flightless birds. Some key differences:
- Ostriches – Very small pectoral muscles less than 1% of body mass
- Cassowaries – Reduced pectoral muscles around 5-8% of body mass
- Penguins – Pectoral muscles adapted for swimming, around 15% of body mass
Without well-developed, strong pectoral muscles and breastbone to anchor them, flightless birds cannot achieve flight. However, their wings still retain enough musculature to allow limited movements.
Feather anatomy of wings in flightless birds
Feathers are key structures required for flight that cover the entire surface of a bird’s wing. Flight feathers attach to the bones and muscles to provide aerodynamic properties. Flightless birds have generally lost most of their flight feathers during evolution. However, they retain covert and downy feathers on the wing surface. The types and extent of remaining feathers varies by species.
Ostrich feather anatomy
Ostriches have completely lost their flight feathers and have bare wings with only small downy plumes in place of large quill feathers. Their wings are not capable of generating any lift or thrust to become airborne.
Cassowary feather anatomy
Cassowaries have wiry quill-like feathers on their wings, but lack the interlocking asymmetrical flight feathers needed for flight. The wing feathers provide protection but no aerodynamic abilities.
Penguin feather anatomy
Penguins evolved feathers suited to swimming in water. Their wings are covered with short, stiff, overlapping feathers. The feathers are flattened and scale-like to become more streamlined underwater. They lack flight feathers, having evolved for propulsion in water instead.
Vestigial wings provide balance and temperature regulation
Although flightless birds cannot fly, their small vestigial wings still serve some functions related to balance, steering, and temperature regulation.
Balance
Wings can be extended to maintain balance while running (ostriches), climbing inclines (emus), or navigating uneven terrain (kiwis).
Maneuvering
Penguins use their wings as flippers to “fly” underwater. Wings provide steering and maneuverability while swimming.
Temperature regulation
Feathers on the wings help retain heat or provide cooling if spread. Penguins can release excess heat by extending flippers. Downy feathers insulate wings from cold and heat.
Other anatomical clues that wings are present
In addition to retained wing bone and muscle structures, other anatomical features indicate evolutionary relationship to flying birds:
- Clavicles (collar bones) – Flying birds have fused clavicles forming the furcula or wishbone. Flightless birds retained unfused clavicles.
- Hollow air sacs – Remnants of air sacs used for respiration during flight are present in some species.
- Reproduction – Flightless birds still lay hard-shelled eggs, an adaptation to flight, rather than soft eggs as in other flightless avian groups.
Conclusion
In summary, flightless birds such as ostriches, emus, cassowaries, kiwis, and penguins do have wings. Their wings retain a skeletal structure related to flight, including bones of the forelimb. However, the wings show significant adaptations in size, musculature, and feathers that make them unsuitable for flight. Rudimentary wing structures provide a clue to their evolution from flying ancestors. So although they will never take wing, flightless birds still provide evidence that wings used to be an integral part of their biology.