Tracing the ancestry of the flightless birds

BABY emus have hatched at Stonehall Farm in Limerick. According to Sinead Shanahan of the visitor farm and RTÉ’s Mooney Goes Wild programme, this is the second year in which the birds have bred; a chick was born during Easter last year.

This year the daddy emu, Eric, sat on 10 eggs for about 50 days and there are now six babies.

The Limerick emus could not be farther from home; these birds are natives of Australia. Not quite as big as an ostrich, an adult emu may be 1.75 metres tall and two-thirds of the weight of an average-sized man.

Like ostriches and the South American rheas, emus are flightless. They still have tiny wings, however, which means that their ancestors were able to fly. Emus may even be flightless for longer than the other big birds; their wings are smaller than those of ostriches or rheas.

Five orders of flightless giants are known to have existed, but only three of them have survived. There were at least 11 species of elephant bird in Madagascar, the biggest birds ever to have existed. Alas, their size didn’t save them; they became extinct about a 1,000 years ago. Half a millennium later, the giant moas of New Zealand were exterminated when the Polynesians arrived there.

There has been much speculation about the origins of the flightless giants. Some experts believe that they are all descended from a few birds of a single species, which flew down from the northern hemisphere many millions of years ago and found a land of plenty. There were lots of nourishing plants for the newcomers to eat but, to be a really effective vegetarian, you have to be big; it takes time to digest plants properly. You need a large stomach with plenty of storage space if hordes of bacteria are to break down the tough walls of plant cells. Vegetarian mammals, from cows to elephants, are large for this reason.

Flying becomes more difficult the bigger a bird gets. However, there were no nasty predators in the vegetarian birds’ new southern home, no big cats or wild dogs. They no longer needed to fly in order to keep out of trouble and so the giants could afford to become flightless.

At that time, Australia, New Zealand, Madagascar and South America were all joined together in a huge island continent called Gondwanaland. Then Gondwanaland broke up into an archipelago of separate islands, which gradually drifted apart, as they are still doing today.

South America would, in time, collide with North America but it remained an island for millions of years. Conditions on the islands changed as their locations shifted in the southern seas and the various flightless birds, adapting to differing environments, became the ostriches, emus and rheas we know today.

Not all experts agree that the big birds had a single ancestor. Birds of differing species may have flown to the southern islands where they evolved independently. Today’s birds, according to this version of events, resemble each other because there were fairly similar conditions in each of the southern landmasses. Recent DNA comparisons seem to favour this second theory.

Whatever their origins, emus ostriches and rheas are not close relatives of each other; if they had a common ancestor, it must have lived a very long time ago. Despite this, however, there are some extraordinary similarities between them. For example, they are all black white and grey. There have also evolved much the same lifestyles. Curiously, their breeding strategies are radically different from those of the large vegetarian mammals.

Most of the vegetarian mammals opt for a polygamous mating system. The dominant males compete for the females and gather their conquests into harems. All the males’ energies are squandered in defeating their rivals. Once they have mated with the females they take no further part in breeding, offering no protection to their vulnerable young.

Only one of the big flightless birds, however, goes in for harem keeping. The South American rheas may have up to a dozen mistresses, all of whom lay eggs in a nest built, or rather dug, by him. Unlike the hoofed mammals, however, he is most attentive to the eggs and young. He does all of the incubating and rearing, while the feckless females go their several ways. Mature male rheas even recruit young male babysitters to help with the household chores.

Ostriches are monogamous in a loose sort of way. A male mates with a chosen hen, who lays eggs in his nest. Then loose-living hens saunter in. They also mate with him and lay their eggs in his nest. Soon there will be eggs from several females. The male and his lawful wife then take turns at incubating the huge pile and looking after the young.

Emus are rather better behaved. They have only one wife per season and all of the housework and child rearing is done by the male. Once the emu female has laid her eggs, he takes over and incubates. Some females will stick around and defend the male on the nest. Others leave and lay clutches with new partners. Once the young hatch, the male becomes aggressive. He will attack any emu, male or female, or indeed a human, who ventures too close to the babies. A most diligent father, he minds his family for about six months.

* Stonehall visitor farm at Curraghchase, Kilcornan, Co Limerick, is open at weekends. www.stonehallvisitorfarm.com.