Its never too early for the Silver-haired and Calif. myotis bats, as Kelly
noted. They -- or at least some of them -- stick around the Puget Sound
region all winter, and come out to feed when the mood/food strikes them.
There are also some records of Big brown bats, mostly specimens in the
Burke Museum collection, that were collected from structures in
W.Washington during the winter months, and there is one under a bridge
here in /Olympia that I've been monitoring this winter, though there's no
evidence that its been coming and going like the other two species. I
can't say that I've ever recorded a Big brown call in the winter, but most
dry non-freezing nights I hear the other two. Saturday night I put out a
detector and recorded both Silver-haired and Calif. bats, after the rain
stopped.

To answer the original question, the bats that seem to leave the area
(incl. Big browns) start showing up in March, though its not until
mid-April that the airwaves become busy. Capitol Lake in Olympia has
thousands of Little brown and Yuma myotis bats feeding over it by early
May, and can be viewed surface-feeding (w/ binocs or scope) on calm nights
in the reflection of the Gov. House hotel from across the lake (around
lamp-post 38). So if you are there birding at dusk during the summer,
stick around another half our or so and you can catch the late show.

If you can see the ears/face (skin) and fur, Big browns have highly
contrasting brown fur against nearly black skin, and the Silver-haired are
dark against dark, and I'd call both species 'chunky.' If anyone has data
or info on Big brown bats here in the winter, I'd love to hear about it,
we're just scratching the surface on winter behavior for bats.

Greg Falxa
Olympia, Wa
gregf at efn.org

Paper on winter bats in W.Wash:
<http://www.cascadiaresearch.org/reports/Bats-NWNaturalist-2007.pdf>

> Date: Sat, 6 Feb 2010 17:34:37 -0800
> From: "Kelly McAllister" <mcallisters4 at comcast.net>
> Subject: Re: [Tweeters] Is it early for bats?
> To: "Ed Swan" <edswan at centurytel.net>, "tweeters"
> <tweeters at u.washington.edu>
>
> Greg Falxa has done research on foraging bats during winter in western
> Washington and found that Silver-haired bats and California Myotis are
> both present in our area during the winter. California Bats are small,
> smaller than your description, I think. Silver-haired bat matches your
> size description fairly well. Big Brown Bats are about the right size but
> I forget whatever I may have heard about their status in winter here.
>
> Kelly McAllister