A science teacher explains: How emus and ostriches lost the ability to fly

The fraternity of flightless birds are today conspicuous in distinct parts of the world, such as penguins in Antarctica, emus in Australia, kiwis in New Zealand, and ostriches in Africa

By Rachna Arora

Birds are incredible animals with locomotion, of which flight is one of their most admirable abilities. Their sleek aerodynamic shape, powerful wings and light body weight allow them to take off and see the world from a different perspective.

While a non-flying bird sounds like an oxymoron, the three largest birds on the planet, ostriches, cassowaries and emus are ground dwellers. About 60 diverse groups of flightless avians exist today with most of them belonging to “Ratites” species. DNA studies have conclusively proved that all flightless birds evolved from ‘flying’ ancestors, having lost their ability to fly over a period of about 10 million years.

The fraternity of flightless birds are today conspicuous in distinct parts of the world, such as penguins in Antarctica, emus in Australia, kiwis in New Zealand, and ostriches in Africa. It is astounding that despite diverse environments, all these varied birds underwent a similar or convergent evolution of being rendered flightless.

Before we address why the birds would no longer want to soar up in the skies, it is imperative to analyse the genesis of why birds fly in the first place. Flight mechanism inherently comes with incredible benefits primarily for escaping predators, hunting and travelling long distances or migrating for better climatic conditions and food. Flying also comes with high costs, consuming huge amounts of energy thereby limiting the body size and weight of the bird. A bird which does not fly conserves energy enabling them to survive on scarcer and also less nutritious food.

Two possible hypotheses, largely influenced by Darwin’s theory of evolution, explains the loss of flight capability in some birds. Changes in the environment, either decreased pressure for flying or increased pressure to develop other traits, made the birds lose their distinctive trait of flying. Steamer ducks found in South American coastal environments are a point in case. The climate is pleasant throughout the year, food reasonably abundant and there is not much fear of predators. Amid little selective pressure to fly, these birds have lost the flying ability over time. For penguins there was an immense pressure to be better swimmers, which provided a greater advantage in terms of food over flying, with the two abilities requiring a tradeoff. Probably nature cannot allow you to be both a great flier and a swimmer!

Likewise, with the extinction of dinosaurs about 65 million years ago, many birds like ostrich and emu facing no challenge from predators made land their permanent base. These birds preferred to nest and feed on the ground and therefore, had a natural disposition to be earth-bound. They gave up flying altogether just over a few generations.

Also, over thousands or millions of years, the birds’ bodies too changed to match this new behaviour. Their bones once hollow became dense, sturdy feathers turned to fluff, wings shrank and in some cases, disappeared entirely. Their keel-like protrusion on their sternums where the flight muscles attach, shrunk or disappeared, except in penguins who repurposed them for swimming. Some birds like ostriches and emus ballooned in size, while others like kiwi remained small. But their legs became thicker, feet sturdier and coupled with strengthened and newly developed thigh muscles, these birds turned into formidable runners.

The role of genetics, particularly regulatory DNA which can increase or decrease the function of specific genes in an organism is being extensively studied in rendering these birds flightless. It is probably the mysterious regulatory DNA that triggered functional changes like strengthening of limbs, changes in wings etc. to make the body of these birds suitable for life on land while keeping their other avian abilities intact.

The highly shrunken wings of the flightless birds are vestigial, though used for other means. The birds can tuck their heads beneath their wings for warmth, or flash them at prospective mates or shelter eggs with them, or even use them to stir as they charge across the plains.

Birds in general are now vulnerable to changes in their environment, triggered by man. Human settlements have already driven many land-bound birds the dodo way. We are breeding our turkeys and chickens to be large and meaty for our consumption and are making them so big that they can no longer fly like their ancestors once did. It is of essence that only nature should drive the process of evolution, dictated by functionality and without the interference of man.

The metamorphosis of flying birds into flightless creatures serves as a wonderful testimony to the power and might of Mother Nature, which has aptly provided both for evolution and devolution mechanisms to cope up with changing demands of existence.

(The writer is PGT- Physics at Shiv Nadar School, Noida)

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