The African Ostrich is the world’s largest living bird, characterized by its immense stature, powerful long legs, and inability to fly. It serves as the standard for a unique group of birds that have adopted a cursorial, or running, lifestyle. When people encounter other species with similar long necks, strong legs, and a terrestrial existence, they often draw comparisons to the Ostrich, prompting a closer look. This shared body plan has led to several species across the Southern Hemisphere to be closely associated with their African counterpart, prompting a closer look.
The South American Rhea
The Rhea is a large flightless species native exclusively to the open grasslands and scrublands of South America, known as the Pampas. There are two main species, the Greater and Lesser (Darwin’s) Rhea, both significantly smaller than the Ostrich. They typically stand between three and five feet tall and weigh up to 66 pounds. Their plumage is distinctly different, appearing shaggier and less structured than the Ostrich’s, and is generally a gray-brown color.
Unlike the Ostrich, which has two toes on each foot, the Rhea has three toes. Although flightless, the Rhea uses its large wings like rudders for balance and steering while running at high speeds. The male bird takes on all incubation duties for a single nest, which can contain up to 50 eggs laid by several different females.
The Australian Emu and Cassowary
Australia and the nearby island of New Guinea are home to two other flightless birds that share the Ostrich’s general body structure: the Emu and the Cassowary.
The Emu is the second-tallest living bird, reaching heights of up to 6.2 feet. It is easily identified by its shaggy, double-layered brown plumage, which provides insulation against the wide-ranging temperatures of the Australian interior. Emus are built for distance running across arid woodlands and savannas, possessing long necks and powerful legs with three forward-facing toes.
The Cassowary, in contrast, is a bird of the dense tropical rainforests of New Guinea and Northeastern Australia. While slightly shorter than the Emu, the Southern Cassowary is the world’s second-heaviest bird, second only to the Ostrich. Its appearance is striking and less Ostrich-like, featuring glossy black plumage, a bright blue and red neck, and a distinctive, helmet-like structure called a casque atop its head. The function of this bony casque is still debated, but it may help protect the bird’s head as it pushes through thick undergrowth.
Shared Traits of Flightless Birds
These geographically separated, yet physically similar, birds—the Ostrich, Rhea, Emu, and Cassowary—belong to a group known scientifically as Ratites. This classification includes all large, flightless birds. Their shared characteristics are a result of convergent evolution, where unrelated species independently develop similar traits in response to similar environmental pressures.
The most defining anatomical feature of all Ratites is the absence of a keel on their sternum, or breastbone. In birds that fly, the sternum features a deep ridge called a keel, which anchors powerful flight muscles. Without this anchor, Ratites are anatomically incapable of powered flight. This lack of flight ability favored an evolutionary path toward strong legs, long necks for foraging, and large size to deter potential predators. Genetic evidence suggests that flightlessness evolved independently multiple times within the Ratite lineage.