text: With the onset of the wet season , when the billabongs and swamps start to fill with water , the night air is filled with the sounds of frogs such as the northern bullfrog and the marbled frog .
simplified: At the start of the wet season , when the billabongs and swamps start to fill with water , the night air is filled with the sounds of frogs such as the northern bullfrog and the marbled frog .