The migration of birds is a phenomenon that has been studied for more than two thousand years. Almost half of the extant bird species undertake migration, covering distances of a few tens of kilometres, up to a calculated 50 000 km per year, sometimes at heights exceeding 9 000 m, at very low temperatures, during the day or night. Some species fly actively and continuously for more than a 100 hours, whilst making efficient use of energy. A variety of strategies and physiological mechanisms are used to perform such feats, but many remain to be studied.
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