The rhea is one of five species of bird which are ratites. Ratites are flightless. There is no keel to the sternum (breastbone). This strip of cartilage or keel acts as the attachment point for the breast and wing muscles attached. Ostriches, cassowaries, emus, rheas and kiwis make up the ratite representatives. The ostrich is the tallest of the ratites. Emus are next tallest but are more lightly built than the cassowary. Rheas are next in size and kiwis are the smallest with five species being about the size of a chicken.
The rhea is a large bird. It is part of the Rheidae family and the Rheiformes family. Paul Mohring gave the genus name of rhea to the bird in 1752 but the rationale behind the name is unclear. Rhea in Greek mythology was known as 'mother of the Gods'.
There are two species of the rhea and a number of subspecies.
Although flightless, the rhea has quite large wings. These are held out when the bird runs and act like sails. Rheas flee from perceived or real danger and run an erratic course. They use the wings like a rudder to maintain their balance as they duck and dive evading their predators. By contrast, the emu has a tiny vestigial wing which is of little use for anything.
If the rhea is cornered it uses its legs to strike out at its aggressor. The three strong toes have hard nails which can inflict nasty injuries. The tarsus of the rhea consists of horizontal plates. The tarsus is the cluster of bones at the base of the toes. Other traits are a strong immune system which helps the rhea recover from injuries and an enlarged section of the cloaca where urine is stored separately.
The Greater Rhea (Rhea Americana) also known as the American, grey or common rhea, has five subspecies. It is America's largest existing bird and can be found in open country from north eastern Brazil to Argentina. For preference, it likes regions with at least some tall vegetation. It is also found in grassland, scrub forest and desert but it avoids humid tropical forest regions.
It may grow to 152cm or more and weigh over 31kg. Like the emu and cassowary, the egg is very colourful. The rhea egg is a golden-green colour. The eggshell fades in direct sunlight. Males may measure 150cm from beak to tail. The plumage varies from greys to browns. Although fluffy, the plumage can appear tattered. Females are generally not as dark as males. The chicks are very attractive with a base colour of grey and having darker longitudinal stripes.
The Greater Rhea is classified as a 'near threatened' species.
The main threats to the greater rhea are ranching and farming. In North America and Europe, the rhea is farmed for its very lean meat. It is also prized for its oil and leather. Rhea leather is finer than ostrich leather as the quill holes are smaller. Craftspeople seek out the eggs for carving. The eggs are large with a lovely natural colour. They are also easy to carve. Local people use the rhea as a source of meat, feathers and leather.
The Lesser Rhea (Rhea pennata) is also known as Darwin's rhea. There are three subspecies. The lesser rhea is regarded as endangered. Most are found in the mountain ranges of Peru south to Patagonia. It frequents altitudes of around 12,000 feet. It grows from 1 to 1.22 metres and weighs 23kg. The egg is a pale green. Sometimes it flees from danger but can also suddenly stop and flatten itself against the ground.
Rheas are omnivorous. Their favourite food is from broad-leafed plants. Seeds, beetles, roots, grasshoppers and carrion are all consumed. They also love tough and spiny vegetation. They don't particularly like cereal crops and farmers welcome the birds which consume beetles and grasshoppers, walking sedately between the rows of crops. Juveniles eat more vegetable matter than do the parents. It seems they have some resistance to poisonous insects such as scorpions. Rheas ingest pebbles to help them grind their food in the gizzard. They are also attracted to items that shine and may swallow car keys and other sparkling items.
The rhea is polygamous with male rheas wooing 2 to 12 females. Once a pair have mated, the male builds a shallow but wide scrape. It is then lined with grass and leaves. Females lay an egg every second day during the breeding season. Once she has laid her eggs, she moves on and mates with another male. The male may sit on up to 60 eggs which have been laid by several different females. On average, seven different females lay 26 eggs in the nest of the male.
The male meanwhile may coax a subordinate male to sit on the eggs while he gathers another harem. The incubation period is 29 to 43 days. Eggs are sometimes left outside the nest. The rhea thinks that predators may take the unattended eggs but leave the nests alone. Regardless of when their egg was laid, all chicks hatch within 36 hours of each other. Once the first chicken cheeps, the others are stimulated to hatch. Males defend their territory and their nests, charging at any perceived threat including humans and female rheas. They will charge at any perceived threat while the chicks are young. Lost chicks may be tended by another male. The families keep in touch with whistle-like sounds. The young are full grown at six months but it will be 18 months before they breed.
When very young and when seeking a mate, the rhea vocalises. Other than that they are silent. When the breeding season is over, flocks of 10 to 100 birds are formed. Flocks formed by the lesser rhea are generally smaller.
Cougars and jaguars are among the natural predators of the rhea. Feral dogs kill younger birds. The eggs are sometimes eaten by armadillos.
Three pairs of rhea escaped in Germany some time ago. These six birds have now become over 100 individuals. These live in a region round the village of Thandorf in Mecklenburg-Vorpommern.