In the autumn before winter, swallows always travel long distances every year-they fly from the north to the far south in droves, where they can enjoy warm sunshine and humid weather, and leave the frost and cold wind of winter to tits, grouse and Thunderbirds who never fly south for winter. On the surface, it is the cold winter in the north that makes swallows leave their hometown for the winter in the south, and then return to their hometown to have children and live and work in peace and contentment in the warm spring in bloom. Is that really the case? Actually, it is not. It turns out that swallows feed on insects and have always been used to preying on flying insects in the air, but they are not good at searching for insect food in cracks in trees and ground. They can't omnivore berries and seeds like rubber grouse and Thunderbird, and they can't eat leaves in winter (conifers don't lose their leaves even in winter). But in winter in the north, there are no flying insects for swallows to prey on, and swallows can't dig the larvae, pupae and eggs of hidden insects such as woodpeckers and woodfinches. The lack of food makes swallows migrate from north to south every year in order to obtain a broader living space. Swallows have become "nomads" in the bird family.
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You can say that children's shoes in Suzhou have no birds.