Simplify the following text for a Grade 4 native English-speaking person.
Original: The fact that no birds appear to have ever weighed much more than half a metric ton suggests that they were restricted in their ability to evolve to larger and larger sizes , and thus in their ability to out-evolve apex predators by sheer bulk as mammals are often able to do ( see Cope 's Rule ) .
Simplified: We know of no bird which weighed more than half a metric ton . Perhaps they could not evolve to larger sizes .