Kathy was right: most bats do leave the area in September, heading for a place to hibernate over the winter. We don’tknow where exactly they go; it is thought they go to mid-altitude caves and abandoned mines in the Cascades, where they can find steady state temperatures of 40-45 to hold them while they hibernate.
One night in February was quite different. A warm Chinook wind had been blasting through the Northwest, and it was 55 degrees or more, pouring warm rain. We were sitting, quietly reading, listening to the rain in the living room. About 9:00 pm we heard these two bats flap & scrabble around. Not just a brief resettling, but they actually worked their way over to the exit site high on the east wall (we had previously figured out where that was) They left for awhile, presumably to stretch their wings and to remember they are animals of flight, to drink, to excrete, and to find meat on the hoof (insects on the wing). We went to bed before they came back, but by the next night we heard them again in the roof. They stayed in our roof all winter.
So yes, Kathy, some bats overwinter here in our area. And from our experience, they mostly sleep. But when the temperatures warm up into the fifties, some bats do leave their warm hibernaculums and venture out into dusk for food, water, and a brief constitutional.
• Bats Northwest
• Bat Conservation International
• Photo by Furryscaly @ Flickr.creativecommons