In December 1967, a flock of thirty wild swans (Cygnus cygnus) flew at a height of 8230 meters from a plane over the Hebrides. The flock came from Iceland and was located at that altitude also by a radar station in Ireland. However, this is a real record: migratory movements generally take place at much lower levels. The surveys carried out in Europe have made it possible to establish that birds in their migratory movements generally fly between 300 and a thousand meters: rarely have been found heights greater than two thousand meters. Many migratory birds, however, exploit thermal currents in their movements, so their flight height is conditioned by the winds and it is not possible to accurately establish average flight quotas.