Rheiformes

Rheiformes is an order of birds that includes the rheas, which are large, flightless birds native to South America. Rheas are ratites, a group of birds characterized by their flat sternum and lack of a keel, which is a ridge on the breastbone that anchors flight muscles in flying birds. There are two extant species of rhea: the greater rhea (Rhea americana) and the lesser rhea (Rhea pennata).

The greater rhea is the largest bird in South America, standing up to 1.5 meters tall and weighing up to 40 kilograms. They are found in grasslands, savannas, and open woodlands across eastern and southern South America. The lesser rhea, also known as the Darwin's rhea, is smaller and inhabits the arid plains and scrublands of Patagonia and the high Andes.

Rheas are omnivorous, feeding on a diet of seeds, fruits, insects, and small vertebrates. They are social birds, often found in flocks, and their breeding behavior is polygynous, with males typically mating with multiple females. Females lay their eggs in a communal nest, and the male incubates the eggs and raises the young.

Fossils of rheas date back to the Pleistocene epoch. The order Rheiformes is characterized by a combination of anatomical features, including a partially ossified sternum, a furcula (wishbone) that is not fused to the sternum, and a large hallux (first toe). While flightless, rheas are capable of running at high speeds, using their wings for balance and display. They are important components of their respective ecosystems, influencing vegetation through their grazing and seed dispersal.