Simplify the following text for a Grade 6 native English-speaking person.
Original: Aepyornis , one of the world 's largest birds , is believed to have been over 3 m tall , weighing close to 400 kg ( 880 lb ) .
Simplified: Aepyornis was the world 's largest bird , believed to have been over 3 m tall and weighing close to 400 kg ( 880 lb ) .