Fact #1: Birds are observed to fly at high altitudes.
Fact #2: High altitude flight is not the shortest possible path, regardless of the model of the shape of the earth you use.
Care to respond to that, Pongo?
The added distance in a flight due to ascending and descending from high altitudes is negligible when compared to the distance added by flying so far at such a high altitudes on a supposed round earth.
How would the flight distance be any different on a flat earth as opposed to a round one?
Pongo still wants to play with unquantified "negligible" quantities, even though the approximate numbers are already in my posts? Maybe it is the effect of the order in which the posts got published...
Assume a trip all around the world (40000 km) at 3 km of altitude above sea level. The trip at sea level is 40000 km (obviously) and the trip at 3 km above sea level is (2 x pi) x ((40000 / (2 x pi)) + 3) = 40000 + (3 x 2 x pi) = 40000 + 18.8 km. The added distance for a trip all around Earth is 18.8 km.
Now, if we take into account that a real migration would be more like 5000 km ( 1/8 of the full circumference of Earth) the added distance would be 18.8/8 = 2.4 km. Now, 2.4 kilometers of additional trip is about 0.05% of the total trip, which is impossible to sense for a bird, and less than the vertical 3 kilometers the bird would have to climb.
As I already said, Pongo should run away from this thread, totally embarrassed by the time he took from all the other contributors by not doing the arithmetic before blurting out this "hypothesis". He seems to have done so, so maybe there is some hope for him after all.