Ratites are a small group of mostly huge, flightless birds. Think ostrich, rhea, emu, and cassowary. Where did they come from? We used to think that they had a common ancestor on the ancient supercontinent Gondwana, and that they dispersed when it began to break up about 180 million years ago.
New evidence suggests ratites are much younger, originating about 65 million years ago. Does that time ring a bell? It’s when the massive Cretaceous-Tertiary (K-T) extinction event occurred. Coincidence? Not really. Because this is the extinction event that took out the dinosaurs, and that was crucial.
Prior to the K-T event, birds and mammals were small, and the primary predators were bipedal theropod dinosaurs like Tyrannosaurus rex and its smaller kin. Following the K-T extinction, a narrow window of low predation pressure allowed birds to become large, and flight was exchanged for running — just what the ratites needed.
Ratites (the name means raft- or flat-chested) lost the keels that anchored the large breast muscles of flying birds. Legs became strong, and running was fast. Ratites also shared a primitive palate structure (paleognath), different from the palates of modern birds (neognath).
It is now believed that flightlessness developed in ratites several times, independently, rather than from a single ancestor. From a distance, the large ratites look similar, but careful examination, including DNA analysis, reveals that they are not closely related. The superficial similarities are due to parallel evolution.
Ostriches, rheas, emus, and cassowaries
Ratites include ostriches in Africa, rheas in South America, emus from Australia, and cassowaries from Australia and New Guinea. Ostriches are the largest. Males reach eight feet tall and 350 pounds, while rheas are about half that size and lighter. Both reside in grasslands or savannahs, where keen vision and fast running are essential defenses. Both can outrun a horse, and some ostriches can hit 30 mph.
Rheas have three toes, while ostriches have two. They’re the only birds with two toes. One of the pair is much smaller than the other and is likely in the process of being lost. Sometime in the future, ostriches will probably run on a single toe, like a horse. The enlarged toe provides an additional means of defense, since the sharp toenail and powerful leg can rip open a large animal, such as a lion or human, with one kick.
Emus are next in size to ostriches, while cassowaries are somewhat smaller. Both emus and cassowaries are very fast runners that can probably reach 30 mph. Both have three toes. The inner toes of cassowaries have sharp elongated nails that are formidable weapons. Emus are restricted to grasslands, while cassowaries prefer forests.
Cassowaries have an elongated boney casque that is thought to protect their head as they break through heavy brush. In addition, three to five long vaneless quills extend along their sides from rudimentary wings. The quills are thought to deflect vegetation when the birds move through the understory.
Moas and elephant birds
Two other giant ratites, moas and elephant birds, need to be considered, even though humans invaded their space and caused their extinction. Moas were the tallest of birds; some reached 12 feet in height. The birds lived in New Zealand, which Polynesian explorers reached in about 1300. Around 100 years later, all nine species were exterminated. Although hunting is thought to be the biggest factor, the newcomers also introduced rats and dogs, which might have eaten eggs and young.
Elephant birds were the heaviest ratites, reaching nine feet in height and 900 pounds in weight. They persisted on Madagascar into the 17th century. Their eggs were about 13 inches long and nearly three feet in circumference and had a volume of two gallons. Humans used the eggshells for cooking.
Kiwis and tinamous
The small and not-so-fast kiwis are an exception to the rule that says ratites are large. Kiwis remained small and adapted a nocturnal lifestyle. They have reduced vision but a keen sense of smell. They spend their days in burrows and forage for earthworms and other organisms on the forest floor under the protection of darkness. Their olfaction is enhanced because their nostrils are located near the tip of their six-inch-long bills. The chicken-size kiwi lays a five-inch egg that can weigh a pound and equal 20 percent of a female’s weight.
There is a second exception to the ratite rule: Tinamous are chicken-size flying birds that have the primitive palate of ratites. They live in South and Central America and Mexico. Their DNA puts them smack-dab in the middle of the ratites. Unbelievably, they are the closest relative of the extinct moas. Truly puzzling.
The origins of the ratites and their relationships are still being debated. Regardless of how such questions are resolved, the ratites are among the most intriguing of all avian groups. Their unique characteristics add to the amazing diversity of birds. — Eldon Greij
A version of this article appeared in the April 2016 issue of BirdWatching.
Three recent articles by Eldon Greij
WHAT BIRDS CAN SMELL
Birds use scents to find food, mates, even their way home.
GRACE UNDER WATER
Adaptations that let birds dive and swim below the waves.
An astonishing array of nests, nest materials, and locations.
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