Wild animals are very sensitive to smells, loud noises, and bright light. Try to discourage the animal by placing a flashing strobe light, a radio tuned to a loud rock station, moth balls or rags soaked in ammonia near the nest. For nocturnal animals such as bats, raccoons, or opossums, use these techniques at night while the animals are out foraging for food. For diurnal animals such as squirrels, use these techniques during the day.
Put a board covered with an old carpet (or with rags tied around it in increments) down into the window well. They will normally climb out on their own at night. If it is still there at night, try turning on the basement lights. The light will usually annoy them and force them to seek the darkness.
(Suggestions by the Chinitimini Wildlife Rehabilitation Center)