Generally we see birds are flying in sky and are moving here and there but some birds move seasonally over long distances to stay for long time and are called Migratory birds.
Migratory birds are those birds which fly out thousands of kilometers over different parts of world in winter to escape the cold, to find best ecological conditions and habitats for feedings, breeding and for the growth of their young ones.
They are important for our ecosystem as they disperse seeds from one place to another and thus maintain the biodiversity along their routes. Guano (droppings of migratory birds) is rich in nitrogen, which is good organic fertilizer. They generally migrate to the temperature or arctic northern hemisphere to breed in summer and migrates south to warmer regions in winter.
They are known to rely on earth's magnetic field to help them migrate the globe. These birds double their weight near the time of migration, so that they can cover long distances without food. They can sleep and migrate at the same time. India receive their migratory birds from Siberia such as Siberian crane, grater flamingo.