S ection 4 49 曲目 49.mp3: Questions: 31-34 Complete the summary of the introduction to a lecture on bird migration below. Write no more than three words and/ or a number for each answer. Birds migrate for two main reasons: to (31) _______________ and to breed. When they are breeding they need to move to areas where they can _______________. In the spring they migrate from (33) _________________ to cooler countries in the north. They spend several months there, flying south again in the winter to (34) _______________.