Rheas are good runners and, surprisingly, have also been seen swimming across rivers, with the wings held high over the back (2) (8). The lesser rhea feeds mostly on plant matter, including grasses and seeds, but also takes some small animals, especially insects (2) (11). Rheas often associate with flocks of grazing llamas, guanacos, vicunas, or sometimes domestic livestock (2).
The breeding season of the lesser rhea typically starts from late July to August (10). Usually living in groups of between 5 and 30 birds, the females separate into smaller groups during the breeding season, while males become territorial, attempting to attract groups of females into a territory, which is defended against rival males (2) (8). The male mates with a number of females, all of which lay their eggs in a single nest, which comprises a scrape in the ground, built by the male and lined with dry grass or twigs. Each nest may contain between 10 and 30 eggs, which start yellowish green in colour, but fade to buff over time. After laying, the females leave and often mate with another male, laying more eggs in a different nest. Unusually for a bird species, it is the male lesser rhea who alone undertakes incubation of the eggs and care of the young. The eggs hatch after an incubation period of around 40 days, and the male leads the chicks away from the nest a few days later, caring for them for a further few months (2). The young reach 90 percent of adult size at around 8 to 9 months, but may not reach sexual maturity until about three years in males, and two years in females (10).