Why do birds migrate? Give an account of different methods of navigation during bird migration. (UPSC 2018, 15 Marks, )

पक्षी प्रवास क्यों करते हैं? पक्षी प्रवास के दौरान विभिन्न नेविगेशन विधियों का वर्णन करें। (Why do birds migrate? Give an account of different methods of navigation during bird migration.)