On the surface, it is the cold winter in northern China that makes swallows leave their hometown to spend the winter in the south. When the spring flowers bloom, they return from the south to have children and live and work in peace and contentment.
Is that true? It’s not. It turns out that swallows feed on insects and have always been used to catching flying insects in the air. They are not good at searching for insect food in tree gaps and ground gaps. They can’t eat berries and seeds and eat leaves in winter like grouse and Thunderbird (conifers don’t fall leaves even in winter). However, there are no flying insects for swallows to prey on in northern winter. Swallows can’t dig out the larvae, pupae and eggs of latent insects like woodpeckers and rosefinches. Due to the lack of food, swallows have to migrate from autumn to spring every year in order to get a broader living space.