Errr, theres some very simple reasons why birds don't fly in a spherical pattern,
Evolutionarily you mentioned that birds came from lizards, lizards that come from the ground, now evolution teaches that lizards did not sprout wings overnight, nor did they learn their migration patterns. So it's entirely plausible that birds keeping to a rough land-path to migratory patterns stems from a long evolutionary chain.
Using a spherical model, most birds would have to pass through the arctic circle/antarctic, meaning it would get considerably colder (I won't even go into the lack of food and unsurvivable temperatures for birds in those places) before it would get any warmer. Now its safe to assume birds aren't that bright, I don't think they understand the concept of the world, hemispheres and seasons, so I don't think they would fly with such foresight, a bird is much more likely to think along the lines of 'if i go north it gets colder, if i fly south it gets warmer'
I could debate the other garbage you mentioned about altitude to birds migrating and how its more efficient on a flat world (its not). but this site is hurting my brain