Simplify the following text for a Grade 5 native English-speaking person.
Original: Most nesting sites are on islands in the northern and southern regions of the Great Barrier Reef . About 1.7 million birds use the sites to breed .
Simplified: Most nesting sites are on islands in the northern and southern regions of the Great Barrier Reef , with 1.4 â `` 1.7 million birds using the sites to breed .