How do migratory birds find their way without GPS?

There are about 10,000 bird species in the world. Almost 20% of them are long-distance migrants. These migratory species fly every year from cold northern countries to warmer southern countries. They do it to find better weather, more food and better survival chances for their nestlings. The size of migratory routes can vary widely, from a few hundred meters to well over 50,000 miles. We, the humans, have been noticing these migratory patterns for more than 3000 years, but have still not been able to crack the mystery totally. We have noted the most popular observations below, but there are still a lot of questions.  

Birds have exert piloting abilities 
Given that migratory paths seem to remain the same year after year, scientists think birds mark out a map to find their migratory destination. Visual markers, particular sounds, distinct smells, and imbibed social cues make up this mapping process. This ability of the birds is often called piloting. Birds literally create a mental map of their migratory pattern, depending on large-scale environment markers. So a bird notices a mountain range and not just a tree. Young birds get some of the map ‘in-built’, i.e. it’s genetically coded, and then their parents help them with practice runs before the long journey.  

Following the stars  
For birds traveling over large bodies of water or tracts of land without discernible features due to rain or snow, another method has been suggested. Some things never change, and the most fixed among them are the orientation of the sun and stars. Scientists have deduced that some species of birds use celestial navigation, just like sailors and explorers used to, and find their way through challenging climate by looking up instead of down. Some birds take to flying at night for this. 

Magnetic field based navigation  
The concept of vision-based magnetoreception means that birds can see the heat generated outlines of magnetic fields, and align themselves in the direction of the field where they want to travel. So, a bird migrating south, will align itself with the outlines of a warmer hill or forest it senses in the south.