As cold-blooded creatures, they rely on outside warmth to live. After the sun sets, the air temperature at higher altitudes is warmer than that nearest Earth because the ground cools faster then the air above it. An area of warmer air forms, known as the inversion layer. To stay within these warmer temperatures, insects climb higher with the rising inversion layer. At sunrise, the insects return to Earth as it becomes warm near the surface.
Birds are warm-blooded, so they have other reasons for high flying.
One is to surmount obstacles like mountains during migration, says Ron Rohrbaugh, an ornithologist at Cornell. Migrating geese, pelicans, and ducks, for example, travel at altitudes between 8,000 and 9,000 feet. And though they don't migrate, bearded vultures have been seen soaring at altitudes as high as 20,000 feet by climbers on Mount Everest. Due to the large volume of mail we receive only questions for publication can be answered. The official Web site of Popular Science Magazine.