Dinosaurs were the dominant land animals for
hundreds of millions of years.
But then, 66 million years ago, an asteroid
hit Earth.
Dust clouds filled the sky and blocked out
the sunlight.
It became much colder.
Plants died. And because it was so cold and
dark, new ones didn't grow very well.
Large volcanic eruptions also added to the
bad conditions.
Most dinosaurs were wiped out, but one group
survived.
These were the avian dinosaurs, which most
of us know as birds.
So why were birds the only dinosaurs that
survived?
There are three main ideas.
Firstly, most birds are small. The ones that
survived the mass extinction of the dinosaurs
were no bigger than ducks.
Small creatures breed faster.
This means they adapt quicker to new situations.
So birds were able to adapt more quickly to
the new conditions than larger animals could.
Birds' small size also gave them another advantage:
they didn't need to eat as much as large animals.
This was handy because food became harder
to find.
A second reason for birds' survival is the
fact they eat pretty much anything.
Birds eat a lot of different things, including
seeds, fruit, insects and even fish.
This is partly thanks to their different beak
shapes.
More and more beak shapes appeared from about
80 million years ago onwards.
A third reason birds survived is because they
could fly.
Flying takes less energy than walking or running
to travel long distances.
So birds could easily escape the bad conditions
and find new food sources and safer places to live.
Today birds are a hugely successful group.
There are at least 11,000 different species.
They are very diverse - just think how different
penguins, sparrows and eagles are.
Birds are found everywhere on the planet - on
all continents and in all environments, from
icy Antarctica to tropical rainforests.
They are incredible creatures that soar above
mountain tops, survive in dry deserts
and even swim in the sea.
