While I am glad you aren't going against evolution, I am rather disappointed in your flow of logic.
You need to ask yourself why birds migrate the way they do. Most birds migrate seasonally. As it gets cold, or before it does so, with the onset of winter, the birds will migrate to a cooler southern climate. And as winter ends and that climate gets to be too hot for them, they move back up north for the cooler northern summers. Now, it is a bit more involved than this (namely, when they start migrating, the factors involved in when and where they migrate, the specific routes they take, and the timing between different species migrating to avoid niches being over filled, among others), but that's a good simple summary.
The point is, birds would not migrate east to west, because it has no or little effect on the climate. It's kindergarten logic to know that if you want to be warmer, you go south. Colder, you go north. East and west only have a notable difference in temperature across incredibly vast distances, or in special geographical areas, where west might be, say, Death Valley.
This is a nice image of sea turtle migration. A lot of it does go north and south. But, right there at the bottom, you see a long migration pattern from South America to (presumably) Australia. This would be the longest imaginable path on your Flat Earth model, and would be the most inefficient of any migration pattern. By your own logic, that shouldn't happen, being so incredibly inefficient. Evolution wouldn't allow it. Unless, of course, the distance around the bottom of the map is actually shorter, being at the bottom of a globe.
Now, to the altitude point. Higher altitudes actually prove to be more efficient thanks to tail winds. And the distance is actually not that much greater. We're talking very small scales. In reality, 500 feet up is not that high. Here is a nice quote from Standford.edu:
"Most birds fly below 500 feet except during migration. There is no reason to expend the energy to go higher -- and there may be dangers, such as exposure to higher winds or to the sharp vision of hawks. When migrating, however, birds often do climb to relatively great heights, possibly to avoid dehydration in the warmer air near the ground. Migrating birds in the Caribbean are mostly observed around 10,000 feet, although some are found half and some twice that high. Generally long-distance migrants seem to start out at about 5,000 feet and then progressively climb to around 20,000 feet.
Just like jet aircraft, the optimum cruise altitude of migrants increases as their "fuel" is used up and their weight declines. Vultures sometimes rise over 10,000 feet in order to scan larger areas for food (and to watch the behavior of distant vultures for clues to the location of a feast). Perhaps the most impressive altitude record is that of a flock of Whooper Swans which was seen on radar arriving over Northern Ireland on migration and was visually identified by an airline pilot at 29,000 feet. Birds can fly at altitudes that would be impossible for bats, since bird lungs can extract a larger fraction of oxygen from the air than can mammal lungs."
I took the liberty to bold the line you would be most interested in. Remember, this is Stanford University. Unless you have a higher education than an Ivy college, I'm going to go with the ones who have studied this first hand much longer and mroe intensively than you have.
Birds are not the most evolved. In fact, that single statement shows you know extremely little about evolution at all, past the cliche phrases that are over used and misunderstood by many who use them. There is no such thing as "most evolved" or even "More evolved." That implies evolution has a goal, something it's trying to reach. It doesn't. If conditions change, species change. And the ones who evolved to better live in the environment will survive. This is not "more evolved" however, because if the conditions changed again, those "more evolved" organisms can suddenly be shit out of luck. You can't even say our ancestors were "less evolved" than we were, because we probably wouldn't have survived in those conditions as we are now. Besides, every species here on Earth is descendant of the dinosaurs. Species evolve from earlier species, so, all species evolved from that time frame. Just like all of the dinosaurs evolved from the species before them, so on and so forth.
Once again another thread that makes not one point that can't be explained or refuted, and, again, shows a profound misunderstanding of the sciences they are attempting to use.