Scientific basis for the migration of swallows from north to south: The advantages of bird breeding in the northern region are that the days in summer are long and the nights are short, and young birds can be fed and nursed in the longer days, which avoids the competition for nesting sites and food when breeding in the southern region with more species. In addition, the number of insects in the northern region is richer than that in the southern region in summer, while there are fewer natural enemies in the northern region, and the predation pressure of natural enemies is relatively low. The winter climate in the northern region is very cold, and it often faces the problem of food shortage. In the south, on the other hand, the breeding season is faced with fierce competition for nest sites and food resources, and the predation pressure of natural enemies is also relatively high, but the climate in winter is mild and the food resources in winter are much richer than those in the north. Migratory birds breed in the north in summer and spend the winter in the south in winter. Both places have advantages. The north here is relative to the tropical areas in the south. In fact, most swallows stay in temperate regions with monsoon climate, not in the very cold north. )