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I remember seeing not long ago that a Griffin Vulture has been spotted at 35,000 feet; but the more commonly Bar Geese regularly exceed 29,000 feet as they fly over Mount Everest on their migration.
Powerful birds which are essentially carnivourus need to fly very high above the ground because they are territorial. The fighter plane which flies at greater height wins. It is easier to attack from heights if there is a possibility of enemy or competitor or contender.They make their nest very high above the ground for the same reason. As animal species down below knows that it is better to finish them when they are young.
Migration in that case must involve traversing distance over diverse geographies under a possible threat from territorial birds.Which I think is the case and due to the threat from carnivorus birds or territorial birds they attempt the migration in groups just like fishes.A coherent groups of small birds attempt the same strategy for defense against Eagles.They make formations to fool or attack the predator.Kind of decpetion tactic.Or these birds fly at an even greater heights to secure against the threat.