Why can't some birds fly? - Gillian Gibb
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.
