You see, their legs are not constructed the same as ours. The arrangement of the leg bones, muscles, tendons and feet is roughly opposite of ours, having sort of a ratchet-type operation. When awake, birds can adjust the leg/foot movement, but when asleep the feet clench tightly, like they're glued to the perch. This happens automatically.
All fall migration flights aren't to the south. Actually, most north Texas birds don't migrate at all. The Grey Bat, however, is a tiny harmless creature that flies north. It lives in warm, dark caves, like south Texas and Florida. But when it wants a place to hibernate each winter, it seeks out relatively cooler areas, perhaps Arkansas or Tennessee.
During this wintertime hibernation in cool areas, its heart rate drops from several hundred beats per minute to twenty or thirty.