Give an elaborate account on the Migration of Birds. Add a note on the problems faced by them during migration. (UPSC 2014, 15 Marks, )

पक्षियों के प्रवास पर विस्तृत विवरण दें। प्रवास के दौरान उनके द्वारा सामना की गई समस्याओं पर एक नोट जोड़ें। (Give an elaborate account on the Migration of Birds. Add a note on the problems faced by them during migration.)