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This is how birds know how and where to fly for winter.
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. For example, the small Sedge Warbler, which we mentioned earlier, weighs normally 10 to 12 grams. Just before leaving for its migratory journey, it weighs twice more!
I could also taste the blood inside my body and feel it, for about the first day. It was the oddest sensation and I have no way of describing how I could taste the blood in my veins, but I could.