Why do birds migrate? ответьте

The fact that migratory birds, living in Europe, migrate to wintering in Africa don't surprise anyone. And that is why on this continent migrating birds living in Asia, affects even scientists: it is much easier for birds would fly to the southern regions of the continent. For example, representatives of passerine annually migrate from the North - East coast of the Pacific ocean on the African mainland, flying over the whole of Siberia. This puzzle is still not solved and, it seems, scientists have to fiddle with it for a long time.

for all birds, one of the principle driving forces behind migration is food scarcity. If all birds were to stay in the same tropical regions year-round food would become scarce and breeding would be less successful. But as food sources are regenerating in the north each spring, millions of birds to those areas, to take advantage of abundance.