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.