Many animals respond to changes in the amount of light available. Their annual migrations, hibernations, mating seasons, and molting times are all triggered by sunlight.

Snowshoe rabbits, for example, completely change their appearance each fall. They gradually begin to lose their brown summer fur and white winter fur grows back in its place. This camouflage makes the snowshoe rabbit almost invisible against the background of the white snow. As the days grow longer in the spring, snowshoe rabbits shed their white winter coats. The white insulating hair of winter falls out and is replaced with darker summer hair. This summer hair is lighter in weight so it is cooler; and it allows the rabbit to blend in with the colors of summer.

The shortening days of fall also alert bears to the approach of winter. The changing amount of sunlight actually stimulates a bear's appetite. Bears must eat a lot in order to gain weight for the long winter ahead. This extra weight helps them store energy for the long winter ahead. Later the sunlight triggers the bears to hibernate. Bears do not enter a true state of hibernation and are somewhat active on a more limited basis throughout the winter months. The lengthening of days in the spring wakes the bears from their long winter rest and they begin the cycle again.

Changing amounts of daylight prompt geese to fly south in the fall and north in the spring. Monarch butterflies, caribou, salmon, and even turtles all respond to the changing length of days throughout each year. Even not so gradual changes in light effect animals. Pet cockatiels are known to be quiet and go to sleep when their cage is covered with a sheet, even in broad daylight.

An eclipse even prompted a flock of chickens to report to the chicken house recently though this change in behavior wasn't exactly welcome; a scientists had just set up his instruments in the chicken house to protect them from the wind during the eclipse. Unfortunately he spent most of the eclipse shooing chickens away.