Why can't some birds fly?

In the lush rainforests of Australia, birds roost in the low branches and amble across the forest floor, enjoying the shade and tropical fruits.

But the jungle isn’t theirs alone.

A dingo is prowling in the shadows, and fruit won’t satisfy his appetite.

The birds flee to safety all but the cassowary, who can’t clear the ground on her puny wings.

Instead, she attacks, sending the dingo running for cover with one swipe of her razor-sharp toe claws.

The cassowary is one of approximately 60 living species of flightless birds.

These earthbound avians live all over the world, from the Australian outback to the African savanna to Antarctic shores.

They include some species of duck and all species of penguin, secretive swamp dwellers and speedy ostriches, giant emus, and tiny kiwis.

Though the common ancestor of all modern birds could fly, many different bird species have independently lost their flight.

Flight can have incredible benefits, especially for escaping predators, hunting, and traveling long distances.

But it also has high costs: it consumes huge amounts of energy and limits body size and weight.

A bird that doesn’t fly conserves energy, so it may be able to survive on a scarcer or less nutrient-rich food source than one that flies.

The Takahe of New Zealand, for example, lives almost entirely on the soft base of alpine grasses.

For birds that nest or feed on the ground, this predisposition to flightlessness can be even stronger.

When a bird species doesn’t face specific pressures to fly, it can stop flying in as quickly as a few generations.

Then, over thousands or millions of years, the birds’ bodies change to match this new behavior.

Their bones, once hollow to minimize weight, become dense.

Their sturdy feathers turn to fluff.

Their wings shrink, and in some cases disappear entirely.

And the keel-like protrusion on their sternums, where the flight muscles attach, shrinks or disappears, except in penguins, who repurpose their flight muscles and keels for swimming.

Most often, flightlessness evolves after a bird species flies to an island where there are no predators.

As long as these predator-free circumstances last, the birds thrive, but they are vulnerable to changes in their environment.

For instance, human settlers bring dogs, cats, and stowaway rodents to islands.

These animals often prey on flightless birds and can drive them to extinction.

In New Zealand, stoats introduced by European settlers have threatened many native species of flightless bird.

Some have gone extinct while others are endangered.

So in spite of the energy-saving advantages of flightlessness, many flightless bird species have only a short run before going the way of the dodo.

But a few flightless birds have survived on mainlands alongside predators aplenty.

Unlike most small flightless species that come and go quickly, these giants have been flightless for tens of millions of years.

Their ancestors appeared around the same time as the first small mammals, and they were probably able to survive because they were evolving— and growing—at the same time as their mammalian predators.

Most of these birds, like emus and ostriches, ballooned in size, weighing hundreds of pounds more than wings can lift.

Their legs grew thick, their feet sturdy, and newly developed thigh muscles turned them into formidable runners.

Though they no longer use them to fly, many of these birds repurpose their wings for other means.

They can be spotted tucking their heads beneath them for warmth, flashing them at prospective mates, sheltering eggs with them, or even using them to steer as they charge across the plains.

They may be flightless, but they’re still winging it.