Swallows spend the winter in the south, not in the south of China, but in Southeast Asia and Australia. One reason why we spend the winter in the south is that the winter in the north is too cold for swallows to bear the low temperature, while the winter in the south is warmer and suitable for swallows to live.
The other is because the swallow's food is insects, but there are few insects in the north in winter, and the food source is not enough, while there is plenty of food in the warm places in the south.
Swallow's flying ability is particularly strong, but its feet are very weak and it is not suitable for activities on the ground or branches. If the swallow stops on a tree with many branches and leaves at rest, it will be blocked by branches and leaves when it is in danger and cannot take off in time. So swallows don't usually stop on trees.
Swallows like to rest in an open place, such as a telephone pole. There is an open space below, so if the swallow wants to fly, just let go of his feet and it will already be in the air. Swallows usually don't land, because they will be clumsy when taking off. However, there are exceptions. For example, if a swallow goes to the pond to build a nest with mud, it will land.
Therefore, swallows only land on the ground when they feel absolutely safe, which is why swallows like to hunt in flight, because it is difficult to take off when landing.