Evolution can go different ways sometimes, and that's why some species become more and more complex with time.
Modern scientists have noticed that birds with complicated skeletons tend to be less biodiverse, so they've started a study.
Here's what they've found.
More about the experiment
A new study about birds and how they change over time suggests that when birds' bones get more complicated, there are fewer types of them.
This means that as birds become more specialized in where they live and what they do, there are not as many different kinds of them.
The study is the first to show a link between how complex a bird's skeleton is and how many different species of that bird there are.
Scientists at the University of Bath looked at 983 kinds of birds and checked how different their wing and leg bones were.
They found that birds with simpler bones, like pigeons and gulls, have more kinds of them.
On the other hand, birds with more complicated bones, like flamingos and ostriches, have fewer kinds.
Birds with complicated bones also tend to live in fewer places and eat in fewer ways.
It might be important
The researchers think that birds with more complicated bones might be in more danger of dying out because they can't adapt as easily to changes in their environment.
They now want to see if this is true for other animals too.
