My Response: ActuaLLy, every time a bird jumps up and flies the earth is sneaky and moves underneath them (at 1,000 mph at the equator) so imagine a bird's consternation at involuntary migration. The bird is going, "MYgration? I didn't ask for this?!?!?"
This is perplexing especiaLLy to birds who think the earth is flat. The birds that fly high enough to notice the curvature of the earth and can handle differential equations and dive bombing for food are a little more accepting of the whole thing.
Just notice neXt time you look at a bird which ones appear to be smiling.
If you don't believe me, just jump up in the air yourself, you'll see you don't come down in the same spot, eXcept for Santa Claus at the North Pole.
Its the original Post Traumatic Stress Syndrome. You leave your post only to come back later to see that it has moved. This eXplains aLL the bird noise, "Where Is My Nest?", thats what the non-smiling ones are saying.