“Even when the bird walks, one feels it has wings.” – Antoine-Marin Lemierre
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.