Bat Usage of a Major Cave System on Northern Vancouver Island

Roosting male long-eared bat, probably Myotis keenii.
Caught at Cathedral Cave, September 7. 1997
(photo credit: Mark Carema)

Mid-Year Progress Report: November 10, 1998

This is a mirrored document from the Island Karst website :

Funded by:

  • British Columbia Habitat Conservation Trust Fund
  • Bat Conservation International

    Martin Davis and Alisa Vanderberg, Island Karst Research
    Trudy Chatwin, Ministry of Environment, Lands and Parks

  • Executive Summary

    This study area on northern Vancouver Island, is the site of the only known cave hibernaculum for any Myotis species in British Columbia, including the endangered Keen's Long-eared Myotis (Myotis keenii). New information regarding critical hibernacula sites and characteristics of these sites is vital to bat management and conservation. The Province of British Columbia is initiating legislation and guidelines to regulate forest practices and ensure protection of forest-dependent endangered species at this time, yet due to the paucity of information for forest-using bats, recommendations are based on information from other jurisdictions.

    In 1996, a study was initiated at the study area caves to determine habitat characteristics of the surface and subsurface environment in order to formulate management guidelines applicable to these species Little Brown Bat (Myotis lucifugus),Long-legged Myotis (M. volans), Keen's Long-eared Myotis (M. keenii), Yuma Myotis (M. yumanensis) within a karst montane coastal forest. In this sparsely populated region, logging practices are the primary management consideration. For this study researchers placed temperature and humidity data loggers within caves and on the surface at three elevation ranges (Low 0-400 metres, Medium 400-600 metres, High 600-900 metres) and within clearcuts versus old-growth environments in order to determine climatic criteria for hibernacula sites and potential impacts of logging over a cave environment in regard to bat usage. The temperature monitoring, combined with a program of bat detection, netting at cave entrances and skull collections (within the caves) is yielding information on types of caves preferred for bat hibernation that is unobtainable elsewhere.

    The hibernation sites are characterized as having stable winter temperatures of 3-4oC and 100% humidity during winter. These sites are deep in caves in the high and mid-elevation range. The winter temperature in caves under clearcuts is not different from that under forested areas. In 1998, we captured 70 bats of five species (Myotis lucifugus, Myotis yumanensis, Myotis volans, Myotis californicus and the Big Brown Bat, Eptesicus fuscus) on 86 net-nights. Summer cave activity involved swarming (a social activity which involves numerous bats flying in and out of the caves) and possible use of a lower elevation cave at 70m as a maternity roost. Hibernation and spring emergence was documented at high elevation caves (600 +m). However, from the swarming and the temperature data we suspect that hibernation may occur at mid-elevation caves (500-600m) under forest and harvested areas. Myotis keenii, a red listed species, uses this drainage for feeding and raising young at low elevation and for swarming and hibernation at mid-high elevations.

    Hibernating long-eared bat, probably Myotis keenii.
    Slot Canyon Cave, 550 metres above sea level, October 31, 1998.
    (photo credit: Martin Davis)


  • To document cave usage by bats in this karst area through discreet visitation, electronic monitoring, guano collection and skeleton collection;
  • To determine the physical characteristics of bat hibernacula through year-round monitoring of temperature and humidity patterns in caves of different elevation ranges;
  • To document summer habitat use patterns of bats in caves and surrounding montane forest;
  • To determine the impact of clear-cut logging on cave physical characteristics and bat use patterns;
  • To document and identify skeletons of bats, marmots, bears and other vertebrates collected in the caves;
  • To make recommendations for conserving bats and bat habitat in the coastal karst environment, both locally and regionally, including conservation of the rare and endangered Keen's Long-eared Myotis.

    Location and Description

    The study area comprises approximately 600 hectares in two adjacent drainages on northern Vancouver Island. This relatively remote area is accessible by a steep trail and ranges in elevation from sea level to 1126 metres. The area is underlain by limestone with over 22 kilometres of mapped cave systems. Both drainages are primarily in the Coastal Western Hemlock Biogeoclimatic Zone, with coniferous forests of sizeable Western Red Cedar, Western Hemlock, and Amabilis Fir. Higher elevations of the study area are in the Mountain Hemlock Biogeoclimatic Zone. Biogeoclimatic subzones include CWHvm1, CWHvm2 and MHmm1. The ecoregion is Western Vancouver Island. Part of the study area has been clear-cut logged up to 25 years ago and has young forest regrowth. A significant portion of the cave area was set aside as Provincial Park, while the remainder is Tree Farm Licence #19.


    Temperature monitoring

    To characterize the physical habitats suitable for bat hibernation we deployed a replicated series of On-set temperature and humidity data loggers within caves and on the surface at three elevation ranges (Low 0-400 metres, Medium 400-600 metres, High 600-900 metres) and within clearcuts versus old-growth environments in order to determine climatic criteria for hibernacula sites and potential impacts of logging over a cave environment in regard to bat usage. The data loggers were placed 10 m. within the cave entrance, deep within the cave (where possible >100 metres from the entrance) and outside the entrance. Data is taken at one hour intervals.

    The 1998 field year began with a check of temperature loggers operating within the caves and at surface sites. All were downloaded and relaunched, and some failed loggers were replaced. In addition, we added one new low elevation site and moved another, to improve the study design. To date, we know that the primary known cave hibernaculum, located at about 800 metres elevation, maintains temperatures between 3-4oC in the winter, with 100% humidity. Temperature data was analysed for logger sites deep in caves as these areas would provide suitable security for hibernating bats. Deep within caves, mean temperatures differed between low, medium, and high elevations with high elevations having significantly lower mean temperatures than low elevation caves. To date there is no significant difference between mean winter temperatures for caves under clearcuts or forested sites. Data processing is ongoing and will be completed this winter

    1998 Mist-net results and "swarming" chronology

    We netted for bats on 32 nights from May 20, 1998 to October 10, 1998 (tables 1-2). There was a total of 86 net-nights and netting efforts focused on the cave entrances in order to document the swarming chronology and to use swarming as an indicator of possible hibernation sites as has been done in Ontario (Fenton, 1969). However, efforts were made to net at all elevations at surface forest sites as well as the caves. The capture ratio (number of bats caught / net-nights) is used as an index of bat use.

    In 1998 we caught 70 bats of five species: Myotis volans (Long-legged Myotis):33 , M. keenii (Keen's Long-eared Myotis):15, M. lucifugus (Little Brown Myotis):18, M. yumanensis (Yuma Myotis):3, and Eptesicus fuscus (Big Brown Bat):1. The latter bat has not been previously captured in the study area. Myotis californicus was netted at high elevation in 1996. Both E. fuscus and M. californicus have not been captured at cave entrances.

    By late July, there is swarming activity (multiple bats flying in and out of caves that are used for hibernation) by male Myotis of 4 species at the mid to high elevation caves. Swarming activity starts at night near 23:30 hours which is much later than forest activity. The swarming activity increased through August and by early September in 1998 it appeared to peak. In 1997 the swarming peaked in mid-August. This annual variation in timing may be attributable to weather patterns as discussed by Fenton (1969). Females and juveniles did not appear with the swarming males until early September. On September 2, 1 adult female Keen's Myotis, and 4 juveniles were caught out of a total of 21 captures. Rain during the mid-September session could account for the lower capture of bats, but it did appear that swarming activity dropped off at this time. The overall capture ratio of bats at cave entrances during the late summer period is 1.51 bats/net night.

    Bat detection

    Bat activity in the caves was monitored by guano collection sheets, ultra-sonic bat detection, actual visits to caves and netting near cave entrances Guano collection sheets were placed or replaced in the caves, to monitor current activity by bats. Return trips are planned in November to pick up guano sheets and check for bats in the caves. Skulls have been collected from the caves since 1995 and are being identified by David Nagorsen, Mammal Curator at the Royal BC Museum (Table 3). Electronic detection for bats at cave entrances used Minibats and new ultra-sonic detectors, constructed locally by Andrew Davis for Island Karst Research using both heterodyne and digital technologies. Netting at cave entrances and various locations near caves was also conducted from May 20, 1998 to October 10, 1998.

    Cave use by bats

    Spring/ early summer (May 20-July 28, 1998)

    We have found that there was little to no activity detected at mid (500-600m) and high (600-900m) elevation caves in early summer. The capture ratio was 0.0 bats per net night over 9 net-nights. Some emergent bats (characterized by roosting/hibernation close to cave entrances) were encountered in four caves, all above 800m elevation. One of these caves, Wormhole, underlies a clearcut. Emergent bats were documented at the cave entrances between May 23 and 27th. However, surprisingly, two bats were found roosting during the day in Wormhole Cave on June 21. This date was later than has previously been observed in the study area. A cool, wet spring may have delayed emergence. There was no recorded activity at low elevation caves, however we only netted for one night at a low-elevation cave during this period.

    Late summer/early fall (July 29-September 15)

    In late summer, bat activity is concentrated at the mid to high elevation caves. The capture ratio is 1.52 bats per net-night. This "swarming" activity was first documented in late July and continued until mid-September. Swarming occurs to varying degrees at most extensive caves above 500 metres elevation. The greatest activity occurs at entrances to the longer caves with the most stable temperatures, such as the Weymer System, Ursa Major System and Wormhole Cave. At mid-elevation caves, most activity is found at entrances which are lower exits to lengthy, deep cave systems that originate at higher elevations and have stable, cool outflows of air, such as the Slot Canyon entrances to the Weymer System. Females of any species have not been found at mid-high elevation caves until mid-August. Their relative abundance for this period is only 3% that of males, deduced from the totals of three years of netting at Weymer. One of the larger pits within the clearcut, Skypot, measuring 15m wide and 25m deep, is used by the bats for feeding (recorded with a digital bat detector). It appears they are feeding at the air thermocline within the pit.

    The capture ratio for low -elevation caves during this period is 0.375. Two significant low elevation caves (Boneyard and Liquid Sky) exhibited no bat activity. However, a 100 metre long cave, Knoll Hole, showed activity during netting on August 11 1998. Three Myotis keenii were captured at the cave and two more long-eared bats were briefly caught, but escaped. Of the three Myotis keenii captured, two were lactating females and one was a juvenile male. Knoll Hole Cave is relatively short (100 metres long) with multiple entrances and some warm sections. Other than this cave, no cave-related bat activity has been detected below 550m elevation. It may be the unusual thermal characteristics of this cave that attract bat use. It appears that female bats may be feeding and raising young at low elevations and then moving to higher elevation caves for swarming and hibernation. Female bats have rarely been caught at the caves at mid and high elevation here and have been netted only later in the summer. This difference in sexual distribution over an elevational and temporal range may be due to a variety of physiological and abiotic factors such as variable energy demands between the sexes, prey availability and climate (Barclay, 1991) (Thomas, 1988). The cave is small, with multiple entrances and wide temperature variations. Some of the upper passages are quite warm and may serve as a maternity colony. Further investigation of this interesting finding is definitely required as this area is slated for forest harvesting in the near future.

    Surface forest habitat use by bats

    spring/early summer

    Some bat use was documented (through bat detection and netting) at all elevational ranges. This year, bats were first seen flying along the littoral zone of Tahsis Inlet in February after an exceptionally warm, sunny day. Subsequent visits and bat detection to Green Creek estuary and the larger Leiner River estuary on June 16, July 7-8, August 10-11, Sept. 4-5 1998 showed that bats feed over the inlet and adjacent estuary. The Leiner River seems to be used as a flyway by Big-Brown Bats (with call frequencies at 20-30khz) to commute to feeding areas over the tidal zone. In a quiet back eddy of the Leiner River, several Myotis sp. bats were detected based on call frequencies at 50-80khz.

    The Green Creek estuary has a small channel that is used by Myotis sp. bats for feeding. One lactating female Myotis keenii was captured at Green Creek on July 8. The capture ratio for low-elevation surface netting is 0.33 bats per net-night. At mid to high elevation there is occasional bat feeding activity in the forest/clearcut margins and marshes (Bogdown bog, Camp pond, Marshswallow marsh and a small marsh near the Weymer headwaters). The capture ratio in the mid-high elevation surface is 0.077 bats per net-night.

    Late summer/early fall

    As late as September 4, calls consistent with Myotis bats were heard regularly over Tahsis Inlet and the adjacent Leiner estuary. There was feeding activity along the forest margins and marshes at high elevation. However no bats were captured at this time.

    Forest versus clearcut bat activity

    In this study we did not attempt to systematically compare bat activity with detectors in forests vs. clearcuts. During opportunistic detection sessions we detected bats primarily in the forest and clearcut margins (approx. 50 passes/ 4 net nights). Bats were also seen flying along trails in the forest, particularly on the section between Deer Drop and Cave 47H. One of the larger pits within the clearcut, Skypot Cave, is used by bats for feeding. Bats do use caves under both old-growth and clearcuts as hibernacula and for swarming.

    Possible new discoveries of hibernacula

    The presence of another hibernaculum is suspected in one of the large caves under the previously logged area. An exploration trip, in conjunction with local cavers, revealed two bats, probably Myotis volans, that seemed to be still in hibernation in mid-June based on their ragged appearance; and the fact that they were covered in moisture droplets. It is also possible that these were emergent bats and were close to an unknown entrance.

    We have also found evidence of cave use by bats on a subjacent ridge. Thanksgiving Cave, an eight kilometre long system, is located between 500-800m elevational range on a ridge, three kilometres southeast of the study area. At the main entrance, we netted for two hours on September 18 and caught two male Myotis keenii. There have been several anecdotal accounts by cavers of bat sightings within the caves here. This cave system warrants further investigation.

    Species Identification

    Due to confusion over species of long-eared Myotis bats in British Columbia, collection of Myotis keenii and wing punches from Myotis volans were requested. These vouchers were collected during mist netting and sent to Dr. Cook in Alaska for DNA analysis.

    Radio telemetry

    Although radio telemetry was removed from the plans for financial reasons, the use of borrowed equipment allowed us to test the technique on two bats in late summer. On Aug. 4th and Sept. 13th two single male Long-legged Myotis were radio-tagged. We picked up the signal of the second bat during the following day, but an exact location was not found before nightfall. The bat was generally located about 500 metres southeast of the cave where it was captured at a similar elevation (700 metres), but was not detected on the following five days of tracking. This technique needs further focused effort.

    Data Analysis

    We have been working on analysis of the temperature data for the 1996-97 and 1997-98 hibernation periods and the summer 1997 period. Data analysis continues.

    Public Reaction

    To date, all public reaction has been positive. Residents of Tahsis, the closest town, are curious about our work and have reported bat sightings to us, including accounts of bats roosting at their homes. Trudy Chatwin has been featured in an article in the Nanaimo Times. Trudy has also made presentations about bats and including this study to Malaspina College Continuing Studies Program and four local elementary schools in Nanaimo. Bat Conservation International's magazine BATS also included mention of this study in the latest issue. The Island Karst website : has been updated to provide results of our study. We are in process of writing a full article on the project for BATS as Bat Conservation International is a funding partner for the study. HCTF has been credited as the major source of funds for the project.


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