The greater rhea feeds mainly on plant matter, including leaves, roots, seeds and fruits. It also takes insects and small animals, such as lizards, frogs, small birds and even snakes, and sometimes catches flies that have gathered around carrion (2) (6) (13). Rheas also commonly swallow pebbles, which help to grind down food in the gizzard, and can often be seen feeding alongside herds of pampas deer, guanacos, or domestic livestock (2) (6). Rheas are good runners, capable of reaching speeds of over 60 kilometres an hour, and, perhaps surprisingly, are also good swimmers (2).
Outside of the breeding season, the greater rhea lives in mixed flocks of up to 30 or more individuals, though old males are often solitary (2) (6) (9). However, at the start of the breeding season, from August to January, breeding females separate into small groups. At the same time, the males become territorial, competing for territories with threats or fights, intertwining necks and biting or kicking at rival males (2) (6) (14). A successful male attempts to attract a group of females into the territory, performing an elaborate courtship display that involves calling, running around the females with feathers ruffled and wings spread, and then standing beside the females with the neck lowered, shaking the wings (2) (6). The nest is prepared by the male, and consists of a depression in the ground, around one metre wide and twelve centimetres deep, lined with dry vegetation and often hidden amongst bushes (2). The group of females all lay eggs in the same nest, so that a typical nest may contain between 20 to 30, or even up to 80, eggs, from up to twelve different females (2) (6) (15).
Unusually, it is only the male who incubates the eggs and rears the chicks, with the females leaving to mate with other males and lay eggs in other nests. The eggs, which are golden yellow but fade to a dull white over time, hatch after an incubation period of 35 to 40 days (2) (6) (9). The male is an attentive and protective parent (2) (9), and, soon after hatching, leads the young away from the nest, and cares for them for another four to six months. The young keep together in a group by means of plaintive contact whistles, and may shelter under the male’s wings if threatened, or if too hot or too cold. The male may even ‘adopt’ chicks that have become separated from other groups (2) (6) (16). The young generally remain together in a group long after the period of parental care is over, until reaching sexual maturity at around two years in females, and three years in males (2) (6) (11).