Migration is vital for surviving
Migration takes place basically in birds to escape from winter and because food become scarcer, this happens usually in the northern hemisphere, they might escape further even until the south hemisphere.
In case of wood Ivis there are evidences that this birds migrate from the Amazon to Orinoco due the inundation that takes place in the shallow areas of the Amazon Valley where they found out their food.
In order to migrate birds follow magnetic field that probably the key in the protein of cryptocrome located in their eyes.
Altitudinal migration is common on mountains worldwide, such as in the Himalayas and the Andes.
For many birds migration is dangerous, because many are hunt for food; however some other birds avoid to migrate by day and they do instead by night having chance to eat by day.
Some other birds prefer to migrate by day because the thermal columns of raising hot air allow them to soar, this happens usually to big winged birds such as vultures, eagles, buzzards, storks.
The birds that most migrate are swallows, geese, some ducks, finches and flycatchers.