– mid-November through mid- March –
(last update 20-Oct-2011)
2004 – 2006 Summary:
During December 2004, two friends from different rural areas in Thurston County contacted me about bat activity near their homes. At a farm near Olympia bats were frequently observed foraging in the early evening on dry nights that were above freezing. During the summer I had identified California myotis bats at this farm, and full-spectrum acoustics confirmed that these foraging December bats were the same species. In south-central Thurston County, small bats were found under shingles during a barn renovation project; a single bat here and there. In the evening bats were observed flying around the periphery of his bright work lights. The bats at this location were also ID’d as California myotis. These reports of winter activity convinced me that I should be monitoring for bats in the winter, since so little information about this behavior was available. Using full-spectrum ultrasonic recorders at locations that I knew had summer bat activity, I found that in addition to the California myotis, Silver-haired bats were quite active throughout the winter, typically on the same evenings I heard myotis activity. I observed both species at some of the foraging locations where I had found them during the summer. Winter & summer foraging patterns were similar; the Silver-haired bats foraging around the upper canopy of large conifers, and the California myotis were more widespread, in the lower canopy, especially along paths, driveways and roads.
Uncertain if the 2004 winter was an anomaly, I continued with more surveying during the winters of 2005 & 2006, and found similar foraging behavior. On one notable evening in late 2005, I heard the final foraging of some Silver-haired bats 5 hours after sunset – the air temp was 6C (43 F) and it was dry and calm when the activity died down along the road through Priest Point Park. I’d been listening to Silver-haired foraging activity since a few minutes after sunset, something that occurs on nearly any dry winter evening when the temperature is above approx. 38 – 40 F (4 C).
The results from this 2004 – 2006 monitoring showed that California myotis and Silver-haired bats were foraging throughout the winter, and were frequently active from November – March. This was great information but I wanted to learn more about the winter life history strategies for these species. In 2007 I started searching for winter roost locations. I occasionally receive reports from individuals when they find a bat here or there, and had a few from the winter months — in a wood pile, behind a bat box, or behind stacked boards in a shed. With this info as a starting point I began frequent searches in likely locations. I located individuals and a few small groups of Silver-haired and California myotis bats. I worked on a search image that includes small gaps behind trim boards on house (like my house), in bat boxes (typically unchecked during winter months), etc. These efforts led to finding bats throughout the winter, all of the ones that could be identified through call recordings or visual observation were California myotis or Silver-haired bats. In early January 2007 someone in the county sent me a photo of 2 Silver-haired and 2 myotis bats roosting under the same sheet of corrugated metal roofing which had been removed during some remodeling of an unheated garage. I was interested in the fact that the Burke Museum (U of W) had a number of bats in it’s collection that were collected during the winter, usually individuals found around homes. Almost all were three species – the two I’d been finding plus Big brown bats. But I had not encountered Big browns in my searching… yet.
2008 – 09 Summary
I expanded searches, and compared notes with colleagues. Jon Lucas had started winter acoustic monitoring at Hanford, and recorded bat activity on subfreezing nights. Greg Green set up a year-round bat monitoring station at a UW wetland near Bothell (north of Seattle). In mid-November 2008 I obsser ed a single Big brown bat roosting under a concrete bridge in downtown Olympia, WA. During subsequent visits (approx. 2-week intervals) this bat was in the same expansion joint until it left in mid-March. The following year a Big brown bat spent the winter in exactly the same location. As of this update (01 Feb. 2011), a Big brown has again been in this same spot since late November. During winters I’ve only observed one other bat under this bridge, and only for a few days during a sub-freezing period. These expansion joints are frequented by night-roosting Little brown and Yuma myotis bats during the summer months. This bridge is 100 meters from Capitol Lake, where during the summer thousands of reproductive female Yuma and Little brown myotis bats have been enumerated arriving to forage, some from as far away as the old railroad pier at Woodard Bay.
2010 – 11 Summary
More physical search and acoustic monitoring effort has been rewarded by the discovery of some groups as well as many individual bats. We deployed new weatherproof Wildlife Acoustics SM2-BAT ultrasonic detectors, and with assistance from an intern and a fellow biologist volunteer, we expanded the number and range of visual searches for roosting bats.
During December 2010 we located Townsend’s big-eared bats day roosting in several structures and under a bridge in Thurston and Grays Harbor Counties. All were withing the same area where we had documented the Townsend’s bats from a ‘new’ colony foraging during spring & summer 2010, in lowland areas of Grays Harbor and Thurston Counties. While searching for Townsend’s in the Chehalis and Black River valleys we found one site where 7 Townsend’s were roosting during a particularly cold week. When the weather soared above freezing, 3 of them left but there were 3 or 4 in the structure until 17 March. An infrared camera we left there taking hourly photos showed them all leaving an hour after sunset on 17 March, a week before I went and picked up the camera. There was no indication that they returned to the structure after the 17th. The camera, a modified ‘trail cam’ type unit, allows us to collect data for up to several months (hourly non-flash photos) without entering the site or disturbing the bats.
Several additional species have been located during visual searches, primarily Silver-haired and small myotis. Most of small myotis appear to be California myotis, but also a few larger long-eared myotis and a few Big browns. A at one rural residence located near the Capitol Forest and the Black River we located 7 Silver-haired and 6 myotis bats (species unknown), tucked into various gaps and crevices, with the majority in a carport, the rest were visible under siding and trim on the house. The volunteer ‘assistants’ have proven to have excellent eyes for locating bats, although I assume we still missed some. Looking around at the other 15 or so houses nearby, I wondered just how many bats might actually be in that half-mile stretch of rural road. A detector will be going down there soon to see what it sounds like there at night. Generally, the detectors we set out in the winter have only occasional ‘hits’ unless you put it somewhere that you already know has bats nearby, like at a night roost. And the super-quiet calls from Townsend’s bats rarely – very rarely – trigger the record function of the call detectors, so like in the summer, physical searches of buildings and visiting with people who have outbuildings seem to produce the results.
Update 01-Feb- 2011: Although recording Townsend’s bats is troublesome, tonight I recorded a pass from one that was close enough to get a decent quality call for species identification using the SonoBat software package of call analysis tools. I was standing behind the church that where we had located a colony last summer. The air temperature at the time of recording (1900) was 34F/1.5C. It would appear that they are not just in the same general area during the winter, but some may be in the same structure that was used for a summer nursery roost in 2010. In general, observations have been similar to the previous 4 winters, and I had a chance to observe movements between roost sites during the multiple ‘cold-snap’ events. One instance was the group of 7 Townsend’s roosted together when the temperature hovered around the low to mid-20sF. (-5 C.) but the group was down to 4 individuals as soon as the daytime temperature rose above freezing.
Others have reported bat activity, one notable report was in late December 2010, from the Port Angeles area — a Townsend’s big-eared bat was hanging around the shop of a fellow who has typically observed 1 or 2 of them day-roosting in his garage and shop during the summer months. He had presumed that these bats left during the winter, but possibly they have just shifted around a bit and are more covert. This December report included an observation of the bat feeding on flying insects above a pile of decaying plant material. Another intereseting report came with a series of photographs showing the arrival of a flying bat at a covered patio, landing on a roof beam, crawling several inches across the beam, then entering a small gap between the butt of the beam and the wall of the house. The location for this report was between Shelton and Olympia, and it was over an hour after sunset when the outside temperature was 19F/-7C, with snow on the ground. Last summer the homeowner reported that she thought that a bat had been coming and going from this same crevice, but it was always seen ‘out of the corner of the eye’ and nowit loosk like she was right. I had recorded a California myotis around this patio last summer, and it would make sense to me that this is what she photographed in December.
Update 28-Feb- 2011: We located a high activity area at an old quarry near the Thurston/ Grays Harbor Counties border where the bat detector recorded hundreds of call sequences during the 2nd week of February. The weather was dry and mild, and 6 species were ID’d over a 4-night deployment. 2 weeks later when the temperature stayed well below freezing for the good part of that last week of February, there were still high activity nights, especially the next-to-coldest night when over 250 call sequences (bat passes) were recorded. Many bats had been found roosting in buildings nearby, so it is possible that the activitiy picked up as it got super-cold because bats were abandoning the unheated buildings and seeking refuge in the crevices at the basalt cliff near the detector site. This activity level is at lest an order of magnitude greater than other sites where we did acoustic sampling during the winter.
Update 01-April- 2011: Winter is officially over, but the bats in the Puget Sound area have yet to shift to their spring behavior. Detectors monitoring for bats at Ft. Lewis are still pretty quiet except at one of the year-round ‘hot spots’ and the Little Brown and Yuma myotis have not started returning to their Woodard Bay maternity roost site.
Other notes on winter bats:
In 2006 Cori Lausen published accounts of Big brown bat capture and tracking effort in extreme winter conditions of Alberta, and collected acoustic data showing that the bats were flying in subfreezing temperatures. Similarly, my data showed that during winter months bats in western Washington were active and flying. Here I targeted areas where I had noted them feeding during summer months, and found even in the winter they were out for prolonged periods. Their behavior – flying a repeated pattern for hours with the presence of ‘feeding buzzes’ in the ultrasonic recordings — indicates that in this area they are foraging and feeding. With persistence, I located night roosts and fresh guano, supporting the winter foraging hypothesis. Inspired by Cori Lausen’s work and with encouragement and advice from Paul Cryan and Michael Baker, these observations became the basis for my manuscript a ‘winter foraging’ piece published in the Autumn 2007 issue of Northwestern Naturalist.
Typically the roosting bats I’ve located during the winter have shown a high degree of site fidelity over long periods. For the past 3 years I’ve observed 1-to-3 Silver-haired bats occuping a standard flat bat box mounted on a garage wall (Jan. 2011 update: 4 Silver-haired bats). I’ve observed bats`in this box year-round since 2007. During winter when the air temperature occasionally drops into the mid-20s(F) or below these bats are absent from the box. They reoccupy the bat box immediately following these short-term sub-freeezng periods of typically 2-5 days. They appear to have alternate locations in the area better suited for bats in sub-freezing conditions, like somewhere warm enough to prevent the large metabolic cost of keeping body temp. above freezing in the rather exposed bat box.
Additionally I reviewed collection records from the Burke and Slater museums. The specimens and associated collection information provided additional clues about winter activity. I found that most of the bats submitted during the winter have been Calif. myotis, Silver-haired, or Big brown bats. The accompanying notes for a specimen typically indicated that the bat was presumed to have been hibernating, but the details often contradicted that conclusion, such as ‘bat flew out from hedge and was captured by grounds keeper’ for one Silver-haired bat that had been collected in Tacoma, WA. The observations didn’t necessarily support the assumption that the bats were disturbed while in hibernation.
Outside the well-documented Townend’s hiberacula, I could not locate any Washington records for more than a just few bats found hibernating at any one location. The museum records indicated that typically only one or a few individuals were encountered. Reports from several concerted hibernacula search efforts conducted by Dr. Stephen Cross, Dr. Clyde Senger, and Mark Perkins during the 1970s – 1990s indicated that predominately Townsend’s big-eared bats were found in the caves and mines of the PNW, with only a very small number of other species found. This pattern seems to be the case for the other western states and provinces, much different from the Midwestern and eastern states where there are/were large hibernation sites occupied by thousands of individuals.
Additional resources related to NW winter bat activity:
Poster presentation at the April 2009 Western Bat Working Group semi-annual meeting in Austin, TX.: Winter, the other bat season: Observations from Washington state. (Jon Lucas & Greg Falxa, 2009)
Article in Northwestern Naturalist: Winter Foraging of Silver-haired and California Myotis Bats in Western Washington (Greg Falxa, 2007)
Paper in Canadian Journal of Zoology: Winter bat activity in the Canadian prairies. (Cori Lausen & Robert Barclay, 2006)
Email contact for Greg: firstname.lastname@example.org