Over the past year I've received reports and dug up info on several locations where bats appear to be overwintering in the Olympia / Thurston Co. area. It was believed by many that bats in North America either hibernated for the winter months in a cave or something, or migrated to a much warmer area, like the Hoary bats that fly to Mexico for the winter.
There have been a few suggestions over the years that some Pacific NW bats occasionally become active or possibly stay active (e.g., David Nagorsen & Robert Brigham, in the 1994 Bats of British Colombia). In 2004 I started getting calls from folks who saw bats flying in December, and 2 of these reports turned out to be California myotis bats, and they were seeing them regularly on dry winter evenings. I started listening for & recording bats that winter, using my trusty Pettersson D24x bat detector, and heard Calif. and Silver-haired bats at most of the locations that I'd heard them in the summer. Not a lot of activity, but quite predictably on mild winter evenings, those non-rainy nights following days that got up to 10 degrees C (50F). I've heard them out and about down to 3C, and for many hours at a location. In Jan. 2005 I listened to several Silver-haired bats foraging at Priest Pt. Park until 11 pm -- 6 hours after sunset! The results of those first couple years of investigating, which included examining the Burke and Slater museum records for bats submitted in the winter (mostly Calif. and Silver-haired bats!) are in a paper I wrote for NW Naturalist in 2007.
I keep gathering information on our winter bats, and have a short list of places to check out this winter, including a barn where the owner regularly saw several Townsend's big-eared bats all last winter, and a bridge that I've seen one Big-brown bat overwinter for the past 2 years, and there's one there again now...
Most of my leads come from 'the general public' so keep me posted!