Head to head
Greater Rhea vs Ostrich
The greater rhea is often called the South American ostrich, and the two do look alike at a distance, both being large, long-necked, long-legged flightless birds of open country. But they belong to different families on different continents, and the ostrich is substantially larger and built for even greater speed, while the rhea has taken the shared strategy of male-only parental care further than almost any bird.
| Greater Rhea | Ostrich | |
|---|---|---|
| Native to | Central South America | Africa |
| Height | About 1.5 m | Up to 2.7 m |
| Weight | 20 to 30 kg | 90 to 130 kg or more |
| Toes per foot | Three | Two (unique among birds) |
| Top running speed | Around 60 km/h | Around 70 km/h |
| Who incubates | The male alone, then raises the chicks alone | Shared: male by night, dominant female by day |
| Egg | About 130 g per egg | About 1.5 kg, the largest egg of any living bird |
| Conservation status | Near Threatened | Least Concern |
Rhea vs Ostrich: common questions
Is a rhea just a small ostrich?
They look similar and fill a similar role, but they are separate lineages. Rheas, ostriches, emus, cassowaries, kiwis, and the extinct moa and elephant birds are all ratites, an ancient group of mostly flightless birds whose ancestors were spread across the southern continents before those landmasses fully separated. The rhea's closest relatives are actually the Australian emu and cassowary and the New Zealand kiwi, not the ostrich. So a rhea is not a small ostrich; it is a South American ratite that independently ended up ostrich-shaped because running fast across open grassland favours long legs, a long neck, and a lightweight body in any bird that gives up flight.
Why does the male rhea raise the chicks?
The rhea has a breeding system in which one male does almost all the parental work. A dominant male courts and mates with a group of females, prepares a scrape on the ground, and the females take turns laying their eggs into it over a couple of weeks, sometimes 20 to 50 eggs from several hens. The male then incubates the whole clutch for about six weeks and leads and defends the chicks for months afterward, becoming so aggressive that he will charge people, vehicles, and even the females. Meanwhile the females move on to lay for other males. Concentrating care in one devoted male, while the females maximise the number of eggs they produce across several nests, is thought to be an efficient use of a short, unpredictable breeding season on the pampas.
Do ostriches really bury their heads in the sand?
No. The image is ancient and persistent but wrong. Ostriches do lower their heads to the ground for real reasons: they turn and tend the eggs in their ground nest with their bills, which from a distance can look like the head disappearing, and a threatened ostrich, especially a sitting one, will sometimes press its head and neck flat along the ground to make itself less visible against the earth. But an ostrich that is genuinely alarmed either runs, and it is the fastest thing on two legs, or, if cornered, fights, delivering a kick powerful enough to injure or kill a lion or a person. It never hides by sticking its head in the sand, which would simply suffocate it.