When the frost gently covers the reeds and the winter sun casts a faint glow, a group of "skywalkers" unfold a dynamic migration scene in the sky with the changing seasons. 

At present, this is the peak of migratory birds, and many bird watching enthusiasts will enter parks and wetlands to quietly observe and record their figures. What is a migratory bird? Why do they want to migrate? How to reach the immigration destination?