Read the passage and answer the question. Every winter the nature club of Bharatpur counts water-birds at Keoladeo. Despite a dense fog last Sunday, the team finished the census. As a result, the warden praised their charts. The teacher, along with two guides, checks the binoculars each dawn. The number of painted storks is written in red ink. Whereas the herons nest in the acacia, the ducks stay on the open water. If the sanctuary remains quiet, the migrants will return in December. The last sentence suggests that the migrants’ return depends on: