Simplify the following text for a Grade 10 native English-speaking person.
Original: Mammals have the advantage of being able to pass salt in solution , mostly in the urine , but kidney function in birds and reptiles is too weak to remove salt in sufficient amounts .
Simplified: Mammals have the advantage of being able to pass salt in solution , mostly in the urine , but kidney function in birds and reptiles is too weak to remove enough salt .