More than 2000 years ago, Aristotle (384-322 B.C.) had already discovered that some birds, like the cranes, were migrating in autumn. Birds fly in a regular, recurrent and seasonal movement from their wintering ranges to breeding areas: nearly 40 % of the bird species living in Europe and Asia are migratory birds, which means that, in autumn, 3 to 4 thousand million birds are leaving Eurasia for warmer places, while other birds arrive from cooler countries.
Birds migrate mainly to search for food. But how do they know it's time to go?
For sure, it's not the lack of food that pushes them to leave, because they leave long before the food comes to miss. In fact, it seems that it's the length of the daytime and the temperature that influence the birds' hormones. That's how, in springtime, the birds "feel" it's time to leave. They then get ready for the migration: they stock "fuel" in order to have enough energy through the whole journey. In fact, during two weeks, they are going to eat a lot more than usual and, thanks to the hormone changes, their body is going to stock these reserves very quickly, gaining mainly grease.
How astonishing are the birds and their body's ability.