Aspects of the Winter Ecology of Bats on Haida Gwaii, British
Columbia
Author(s): Douglas W BurlesM Brock FentonRobert MR BarclayR Mark
BrighamDiane Volkers
Source: Northwestern Naturalist, 95(3):289-299. 2014.
Published By: Society for Northwestern Vertebrate Biology
DOI: http://dx.doi.org/10.1898/12-32.1
URL: http://www.bioone.org/doi/full/10.1898/12-32.1
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NORTHWESTERN NATURALIST
95:289–299
WINTER 2014
ASPECTS OF THE WINTER ECOLOGY OF BATS ON HAIDA GWAII,
BRITISH COLUMBIA
DOUGLAS W BURLES
Gwaii Haanas National Park Reserve and Haida Heritage Site, Kamloops, BC V2B 8A8 Canada;
[email protected]
M BROCK FENTON
Department of Biology, University of Western Ontario, London, ON N6A 5B7 Canada
ROBERT MR BARCLAY
Department of Biological Sciences, University of Calgary, Calgary, AB T2N 1N4 Canada
R MARK BRIGHAM
Department of Biology, University of Regina, Regina, SK S4S 0A2 Canada
DIANE VOLKERS
Banff National Park, Banff, AB T1L 1K2 Canada
ABSTRACT—Bats of the temperate region of North America avoid winter by some combination of
migration and hibernation at a location that provides the right conditions for minimizing energy
expenditure over winter. Such optimal conditions are commonly found underground, and most of
the best known hibernacula occur in caves or mines. Where winters are milder, some bat species
hibernate in hollows in trees. Haida Gwaii, British Columbia, has the dual distinction of being
relatively far north in terms of bat distribution, but with a relatively moderate oceanic climate. We
hypothesized that because of the moderate winter temperatures, hibernating in trees could be an
option for bats on Haida Gwaii. We used data loggers to monitor temperatures inside potential
roost trees during 9 winters between 2002–2003 and 2012–2013. We found that mean winter
temperature inside the trees ranged from 2.3–6.56C, and in most years temperatures either did not
drop below freezing or else did so only for short periods of time. We calculated that a 6 g bat
would have required between 2.49–2.92 g of fat to hibernate in the tree roosts that we monitored,
which is well within the limits that Little Brown Myotis (Myotis lucifugus) are known to accumulate
in autumn. Our acoustic data demonstrated that California Myotis (Myotis californicus) were
periodically active during all winter months except December, which we view as evidence that this
bat hibernates locally either in trees or buildings. The absence of Little Brown Myotis, Keen’s
Myotis (Myotis keenii) and Silver-haired Bat (Lasionycteris noctivagans) observations during winter
suggests that they may either use more common hibernacula, such as caves, or migrate off the
islands.
Key words: British Columbia, California Myotis, Haida Gwaii, hibernation, Keen’s Myotis,
Little Brown Myotis, Silver-haired Bat, winter bat activity
For insectivorous bats in temperate regions,
winter is a time of food shortage, reflecting the
general lack of activity and availability of insect
and other arthropod prey. Bats of the temperate
zone in much of North America avoid winter by
some combination of migration and hibernation
(Fenton 2001). In most cases, the locations that
bats choose as hibernacula provide conditions
for minimizing over-winter energy expenditure
and water loss. These conditions vary somewhat depending on species, but generally
include cool, stable temperatures with high
relative humidity (Altringham 1996). For small
bats, such as Little Brown Myotis (Myotis
lucifugus) hibernating in Kentucky and New
Jersey, the optimal temperature of hibernacula
289
290
NORTHWESTERN NATURALIST
is approximately 26C (Hock 1951; McManus
1974), and any variation from this temperature
results in an increase in metabolic rate. Stable
temperatures also have an influence on hibernation success as frequent temperature fluctuations cause more frequent arousals resulting in
greater energy expenditures (Ransome 1971;
Twente and others 1985). High humidity at
hibernation sites is also an important factor as
water loss due to metabolism occurs even in
deep torpor, and the rate of water loss is
thought to influence the duration of bouts of
torpor (Thomas and Geiser 1997).
For some populations of bats, optimal conditions for hibernation are found underground,
and most of the documented hibernacula are in
caves or mines. For example, in much of their
geographic range, Little Brown Myotis usually
hibernate underground in caves or mines
(Fenton and Barclay 1980). In more southerly
regions of North America and Europe, some
species of bats hibernate in hollows in trees
(Altringham 1996). In eastern Canada and the
United States, hibernation in underground
locations exposes bats to white-nose syndrome,
which has decimated some bat populations
since its arrival in 2006 (Warneke and others
2012). Less is known about where bats hibernate
in western North America, and no large
hibernacula are known west of the Rocky
Mountains. In southeast Alaska, over 400 caves
have been investigated with only small numbers of bats being recorded in a few caves
(Lewis 1997; K Blejwas, Alaska Department of
Fish and Game, pers. comm.) In British Columbia, bats have been reported hibernating in .60
caves or mines, but invariably only small
numbers have been observed (DW Nagorsen,
Mammalia Biological Consulting, pers. comm.).
On Vancouver Island, for instance, where caves
have been extensively surveyed, Davis and
others (2000) found evidence of bat activity in
15 of 31 caves investigated, but observed only
small numbers of myotis (up to 45 individuals)
in 5 caves. The only exception appears to be a
mine in the east Kootenays that may support in
excess of 100 hibernating bats (CL Lausen,
Wildlife Conservation Society, pers. comm.).
Similarly, in the northwestern United States,
only small numbers of bats have been found in
a few caves or mines (Perkins and others 1990;
Kuenzi and others 1999).
95(3)
Haida Gwaii (formerly the Queen Charlotte
Islands), British Columbia, Canada, at 52–546N,
has the dual distinction of being relatively far
north in terms of bat distribution, but with a
relatively moderate oceanic climate. Sandspit,
for instance, which is 6 m ASL and ,1 km from
the ocean, experiences an average of 330 frostfree d/y (Canadian Climate Normals 1993).
Haida Gwaii is separated from southeast Alaska
to the north by the 60 km-wide Dixon Entrance
and from mainland British Columbia to the east
by the 60 to 100-km-wide Hecate Strait. The
archipelago may be sufficiently isolated that
few, if any, bats migrate from the islands to
overwinter elsewhere.
Four species of bats occur on Haida Gwaii.
Keen’s Myotis (Myotis keenii) is relatively rare,
known from only a few locations (COSEWIC
2004). The only known roosts are in heated rock
crevices associated with a hot spring (Burles
and others 2008), although they likely also
occasionally roost in trees and in buildings, as
elsewhere (Parker and Cook 1996; Mackay and
others 2000; Davis and others 2000; Boland and
others 2009). Little Brown Myotis and California
Myotis (M. californicus) are both relatively
common. On Haida Gwaii, Little Brown Myotis
have also been found roosting in heated rock
crevices, as wells as in trees (Burles 2001). Little
is known of roosting habits of California Myotis
on Haida Gwaii, but elsewhere they roost in
hollows, cracks, and crevices in trees (Brigham
and others 1997). Silver-haired Bats (Lasionycteris noctivagans) are relatively uncommon on
Haida Gwaii. They generally roost in hollows in
trees or under pieces of loose bark (Parsons and
others 1986; Barclay and others 1988; Betts
1998). Of the 4 species, the Silver-haired Bat is
the only species known to be capable of flying
across large bodies of water (McGuire and
others 2012) and thus could potentially migrate
to the mainland.
We studied aspects of the winter ecology of
bats on Haida Gwaii. Information on winter
activity and hibernacula are lacking for much of
coastal British Columbia and adjacent Alaska.
We hypothesized that because of the moderate
winter temperatures experienced on Haida
Gwaii, bats could potentially hibernate in trees.
Based on this hypothesis, we made the following predictions: (1) winter temperatures in
hollows and spaces under bark of trees will be
WINTER 2014
BURLES AND OTHERS: WINTER BAT ECOLOGY ON HAIDA GWAII
mainly .06C; and (2) tree hollows will be more
useful as hibernacula for bats than spaces under
bark. We also acoustically monitored for bats
throughout the winter to determine species
presence and activity.
METHODS
Potential Hibernation Trees
We used Ibutton Thermocron data loggers
(Dallas Semiconductor Corp., Dallas, Texas) to
monitor temperatures in crevices or hollows
and under the bark of potential bat roost trees to
determine if it was feasible for bats to hibernate
in trees on Haida Gwaii. Although the data
loggers are rated by the manufacturer as being
accurate within ± 16C, we first confirmed this
by placing 1 data logger next to the weather
instruments at the Sandspit airport weather
station (YZP, 53.2506N, 131.8126W) and set it to
record hourly for a week. We then compared
the temperatures it recorded with those recorded by the weather station and calculated the
average difference in the temperature. This
average difference was later used as a correction
factor for all temperatures recorded by this
device during experiments. All other data
loggers used in our study were calibrated with
the original data logger to minimize differences
among devices, as well as between the devices
and YZP temperatures.
During winters 2002–2003 and 2003–2004, we
monitored ambient temperatures in 2 known
and 1 potential roost tree located on Huxley
Island (52.4416N, 131.3636W). Because it was
logistically difficult to get to Huxley Island, in
2004 we shifted our observations to Skidegate
Inlet (53.2266N, 131.9456W), choosing another 3
potential roost trees near the shoreline where
we thought that proximity to the ocean would
moderate ambient temperatures. Beginning in
2006, we focused all our efforts on 1 tree with a
deep cavity. In 2005–2006, 2010–2011, 2011–
2012, and 2012–2013 we placed data loggers
both inside and under the outer bark of 1 of the
study trees (SI #1) to assess whether a bat could
hibernate under the bark. In each case, the data
loggers were programmed to record temperatures every 2 or 3 h between 1 November and 30
April the following year. We also obtained daily
temperature records for winter months from
2000–2001 to 2012–2013 from YZP as a general
measure of ambient temperature for the area.
291
Mean winter temperature, standard deviation,
and minimum and maximum temperatures
were determined for each data set.
Winter Energetic Modeling
To further explore the feasibility of bats
hibernating in hollows or crevices in trees on
Haida Gwaii, we used the formulas of Humphries and others (2006) to determine how
much fat would be required for a Little Brown
Myotis to hibernate in trees, with one exception.
Rather than using the metabolic rate for torpor
(TMR) of a bat hibernating at 26C (the lower
ambient set point temperature, Ts, or temperature at which TMR is least) of 0.03 ml O2/g/h as
determined by Hock (1951), we chose to use the
TMR of 0.02 ml O2/g/hr as calculated by
Thomas and others (1990a). We chose this value
because Jonasson and Willis (2012) found that it
better predicted actual energy expenditures of
hibernating bats in Manitoba than did Hock’s
estimate of TMR. Thus, when Ta exceeded Ts,
the energetic cost of torpor (Etor) was calculated
as Etor 5 TMR*Q10.(Ta-Ts)/10
The value for Q10 (the change in torpor
metabolism resulting from a change in ambient
temperature, Ta) was calculated as 1.6 + 0.26 Ta
2 0.006 Ta. When Ta , Ts, Etor was calculated as
Etor 5 TMR + (Ts 2 Ta)Ct. In the latter formula,
torpor conductance (Ct, a measure of the ease
with which heat flows between the animal and
the surrounding air) was set at 0.055 ml O2/g/
6C. The above formulae allowed us to calculate
the number of ml of O2 a bat would consume
during a specific time period at a specific
temperature. Given that 1 ml of O2 is equivalent
to the consumption of 20.1 Joules (from fat), and
that fat contains 39.3 kJ/g, we could then
calculate the amount of fat consumed under
those conditions.
Using the temperatures recorded in our
experimental trees we calculated the amount
of fat required for a 6 g bat, the approximate
mass of Little Brown Myotis in summer on
Haida Gwaii (Burles and others 2009), to
hibernate in each of our study trees from 1
November to 30 April the following spring. To
determine this we calculated Etor for each 2–3 h
interval between 1 November and 30 April each
year, and then summed all calculations to
estimate overall winter costs of torpor. We
also used mean winter temperatures from the
292
NORTHWESTERN NATURALIST
Sandspit airport to calculate fat requirements for
hibernation for winters 2002–2003 to 2012–2013
to see if this more generalized estimate of fat
requirements was comparable to our calculations.
As our model does not take into account the
energetic costs of arousals, we used Thomas and
others’ (1990b) estimate of 108 mg of fat
consumed during a 3-h arousal and an estimate
of the average torpor bout lasting about 13 d
(Twente and others 1985; Jonasson and Willis
2012), or 14 arousals per winter, to determine
the energetic costs of arousals as 1.51 g per
winter. This figure was added to each estimate
of the winter cost of torpor to determine the
overall amount of fat a bat would require to
survive each winter.
Winter Acoustic Surveys
We passively monitored bat activity in a
residential area of Sandspit during winter
months on an opportunistic basis (when temperature was .06C, and there was little wind or
precipitation) using a bat detector. In 2000–2009,
we used a broad band Anabat II detector and
delay switch with a clock (Titley Electronics,
Ballina, Australia) connected to a cassette tape
recorder (CTR-96; Radio Shack, Fort Worth, TX).
In 2011 and 2013, a Song Meter SM2BAT auto
recording system (Wildlife Acoustics, Concord,
MA) was used. All recordings were tentatively
identified to species on the basis of echolocation
call structure. Call parameters (primarily call
duration, minimum and maximum frequencies,
and call slope) were measured using Anabat5
(Titley Electronics, Ballina, Australia) and BatSound Pro (Petterson Electronik, Uppsala, Sweden), and then compared with those obtained
from field recordings of known bats (D Burles,
unpubl. data), or by making visual comparisons
using Sonobat (Arcata, California). Specifically,
California Myotis have short-duration calls of
2.8 ± 0.2 msec that sweep from 92.3 ± 4.0 Khz
down to a minimum frequency $44 ± 1.2 Khz
(a 5 0.05, n 5 30), while Little Brown Myotis
have relatively short-duration calls (3.6 ±
0.2 msec, a 5 0.05, n 5 63) that sweep from
about 76.1 ± 2.8 Khz (a 5 0.05, n 5 63) down to
a minimum frequency of 36.8 ± 0.6 Khz (a 5
0.05, n 5 63). Keen’s Myotis calls are similar to
those of Little Brown Myotis except that they
are of shorter duration (2.8 ± 0.2 msec, a 5 0.05,
n 5 48) and have a higher maximum frequency
95(3)
(96.4 ± 4.0 Khz, a 5 0.05, n 5 48), which results
in a call with a much steeper slope (23.3 vs.
11.3 Khz/msec). Silver-haired Bats have relatively long-duration calls of 10.4 ± 1.1 msec that
sweep from about 36.4 ± 2.7 Khz down to 25.7
± 0.4 Khz (a 5 0.05, n 5 10).
Radio-telemetry
We used radio-telemetry to locate the roosts of
Little Brown Myotis at 2 locations in Gwaii Haanas
National Park Reserve and Haida Heritage Site –
Gandll K’in Gwaayaay (52.5766N, 131.4436W) and
Huxley Island. Between 10 to 24 September 2001,
we captured bats using 6-, 9-, or 12-m long mist
nets as they emerged from roosts near the hot
springs on Gandll K’in Gwaayaay or flew along
the shore on Huxley Island. Our assumption was
that by September bats would be congregating
near where they were going to hibernate and we
might be able to track them to their hibernacula.
We set a harp trap (Tuttle 1974) in an effort to catch
bats along flyways or at the entrance of a mine adit
which might have served as a night roost. We also
recorded the echolocation calls of flying bats using
the high-frequency output of a Pettersson D980
(Pettersson Electronik AB, Uppsala, Sweden)
detector through an Ines DAQ i508 high-speed
card to a PC running BatSound Pro.
Radio tags weighing 0.45 g (LB-2; Holohil
Systems Ltd., Carp, ON) were glued to the
backs of selected bats using surgical cement
(Skin Bond; Smith and Nephew Inc., Mississauga, ON) after the fur had been trimmed. Bats
were chosen to carry a radio tag on the basis of
non-reproductive status and their size (mass
.5 g and relatively large wings). Gluing on
transmitters ensured that they would be shed as
the hair grew back. The bats’ activities were
then monitored using a radio receiver (Suretrack STR 1000; Lotek Engineering Inc., Newmarket, Ontario; or a TRX 2000S: Wildlife
Materials Inc., Murpheysboro, IL) with a twoelement directional antenna. Radio-tagged bats
were generally followed for at $1 hour immediately after release to ensure that the bat
remained active. On the following days, the
area was surveyed by boat and on foot to locate
where the bat was roosting. Whenever possible,
the exact tree or rock crevice roost was
pinpointed. Once a roost was identified, it was
monitored the following evening for time of
emergence and number of bats emerging.
WINTER 2014
BURLES AND OTHERS: WINTER BAT ECOLOGY ON HAIDA GWAII
293
TABLE 1. Summary of locations and roost types on Haida Gwaii, British Columbia, in which winter
temperatures were monitored to determine the feasibility of bats hibernating at each location.
Site
Latitude/
Longitude
Tree type
HU-cave
52.4446
–131.3706
Cave
HU #1
52.4386
–131.3626
60 cm dbh Sitka Spruce
snag with no bark
HU #2
52.4396
–131.3636
1.5 m dbh Western
Redcedar
HU #3
52.4386
–131.3686
1.5 m dbh Western
Redcedar
SI #1-inside
53.2266
–131.9376
2 m dbh Western
Redcedar
SI #1-bark
53.2266
–131.9376
2 m dbh Western
Redcedar
SI #2
53.2066
–131.9736
SI #3
53.2456
–131.8216
2 m dbh Sitka Spruce
snag in advanced
decay
50 cm dbh western
hemlock snag
RESULTS
Conditions of Potential Hibernation Sites
Between 2002–2003 and 2012–2013, we obtained 18 temperature data sets during 9
winters from 2 used and 4 potential bat roost
trees at 2 different locations on Haida Gwaii,
and 1 cave known to have been used by bats
(Table 1). One data set (SI #3) was excluded
from our analyses because at some point during
the winter the data logger was ejected out of the
cavity onto the ground.
Temperatures inside all the trees we monitored remained well above freezing for most of
the time in all winters (Table 2). In 5 data sets
temperatures did not drop below 06C (Table 2),
while in a further 8 sets temperatures dipped
below freezing for only short periods of time (1–
9 d). Temperature data from YZP (Table 3)
revealed that the winters during which we
monitored conditions were generally typical of
conditions experienced since 2000, with the
exception of winter 2002–2003, which was
considerably warmer. In the 1 winter that we
had a data logger inside the Huxley cave,
Roost type
Distance from
ocean (m)
Elevation
(m ASL)
500
120
15
10
100
50
300
80
5
5
5
5
1200
30
400
10
Cave - 50 m in
from entrance
where bat
guano found
25 cm inside
crevice 3 m
above ground
75 cm inside
hollow 3 m
above ground
20 cm inside
hollow 1.5 m
above ground
75 cm inside
hollow 1.5 m
above ground
Under loose bark
1.5 m above
ground
20 cm inside
hollow 1 m
above ground
25 cm inside
woodpecker
cavity 5 m
above ground
temperatures averaged 7.56C and varied between 6.5 to 8.06C. Temperatures under the
outer bark of SI #1 were considerably lower than
those inside the tree, with greater extremes and
variability (Table 2). The wood of the tree thus
appeared to have a buffering effect on temperatures inside the tree. Temperatures inside the
study tree were, in turn, considerably cooler in
both years than temperatures recorded at the
airport.
Winter Energetics Modeling
From the temperatures we recorded, we
calculated that a 6-g Little Brown Myotis
hibernating in our study trees would require
2.49 to 2.92 g (n 5 12) of fat to survive the winter
(Fig. 1). Our 3 estimates for a bat hibernating
under the bark of SI #1 varied between 2.58 and
2.89 g (n 5 4). Our highest estimate of fat (2.93 g)
required was for a bat hibernating inside the
cave on Huxley Island, which was almost
identical to our estimate for nearby HU #1
(2.92 g). Based on the mean winter temperatures
for YZP since 2000 we estimated that the
294
NORTHWESTERN NATURALIST
95(3)
TABLE 2. Summary of temperatures recorded at potential bat hibernation sites located on Huxley Island and
in Skidegate Inlet, Haida Gwaii, British Columbia, between 1 November and 30 April, during 9 winters, 2002–
2013. Recording intervals were every 2 h in 2002–2003, 2004–2005, and 2005–2006; every 2.5 h in 2003–2004; and
every 3 h thereafter (n $ 1212 in all datasets.). *data missing for short time periods; +data logger fell out of cavity
part way through the winter.
Study location
Period monitored
Mean 6C
Standard deviation
Max 6C
Min 6C
2002–2003*
2003–2004
2003–2004
2003–2004
2002–2003*
2004–2005*
2005–2006
2006–2007
2007–2008*
2010–2011
2011–2012
2012–2013
2005–2006*
2010–2011
2011–2012
2012–2013
2005–2006
2005–2006+
6.5
5.1
4.9
5.2
7.5
4.6
4.3
3.7
3.1
3.5
2.6
3.9
4.1
2.7
2.3
3.6
3.7
4.5
2.7
2.1
2.1
1.6
0.4
1.8
1.7
1.6
1.8
1.8
1.9
1.1
2.5
2.8
2.7
2.0
1.4
2.6
11.7
9.7
10.0
8.5
8.0
7.2
8.5
8.0
8.0
8.6
6.8
6.3
11.5
12.1
9.7
9.5
8.2
11.0
25.8
20.3
22.5
0.0
6.5
0.2
0.5
22.0
20.5
20.7
25.8
1.3
23.0
24.6
210.3
21.3
1.2
22.0
HU #1
HU #2
HU #3
HU-cave
SI #1-inside
SI #1-bark
SI #2
SI #3
amount of fat required for a bat to overwinter in
Sandspit would have been between 2.50 to 2.75 g
(n 5 13).
Winter Activity Monitoring
We acoustically monitored bat activity near
Sandspit on 157 nights for a total of 674.1 h of
recording (Table 4), during the winters of 2000–
2001 and 2012–2013. Bat activity was recorded
on 66 occasions during all winter months except
December. Ambient temperature ranged between 5 to 126C on nights that bat activity was
recorded. California Myotis were by far the
most common and were active throughout
winter. The only collection of a bat in winter
at Sandspit was a California Myotis freshly
killed by a cat on 26 February 2005 (D Burles,
unpubl. data). Little Brown Myotis were detected as late as 25 October and as early as 17 April
but were not recorded in between. Similarly,
Silver-haired Bats, which were regular visitors
at this site in August and September, were
not detected between late September and late
April. Keen’s Myotis, which were occasionally
TABLE 3. Summary of winter temperatures (1 November to 30 April) recorded at the weather station (YZP) at
Sandspit, British Columbia, 2000–2013 (n $ 168 in all data sets.)
Winter
Mean winter
temperature
(6C)
Number of days
temperature
,0.06C
Maximum number
of consecutive days
temperature ,0.06 C
Minimum winter
temperature (6C)
Maximum winter
temperature (6C)
2000–2001*
2001–2002
2002–2003
2003–2004
2004–2005
2005–2006
2006–2007
2007–2008*
2008–2009
2009–2010
2010–2011
2011–2012*
2012–2013*
Means
5.1
4.2
6.0
5.1
5.4
4.7
4.3
4.1
3.8
5.1
4.3
4.0
5.0
4.7
25
29
10
13
14
28
22
11
42
24
35
42
24
25
1
3
3
1
2
1
2
2
2
1
1
4
1
1.7
25.0
25.0
28.3
25.0
26.1
25.6
27.2
26.1
27.0
215.1
24.8
212.0
22.5
26.9
12.9
12.8
13.9
15.0
15.0
12.8
11.7
11.7
13.0
11.6
13.2
13.5
13.4
13.1
* Incomplete data record.
WINTER 2014
BURLES AND OTHERS: WINTER BAT ECOLOGY ON HAIDA GWAII
295
FIGURE 1. Summary of the number of grams of body fat required for a 6 g bat to hibernate in a number of
locations on Haida Gwaii, British Columbia. Calculated values are based on temperatures recorded inside a
Sitka Spruce snag (HU #1-%); 2 Western Redcedars (HU #2+ HU #3- ) and Huxley cave (D) on Huxley Island;
in a Western Redcedar (SI #1 inside-e, under bark-X) and in a Sitka Spruce snag (SI #2-#) near Sandspit; and at
YZP (+) as calculated using formulas described in the text. Mean winter temperatures for YZP (—) for the period
2000–2013 are also shown.
recorded at the site during summer, were only
recorded once during winter, in late April.
Feeding buzzes (attacks on flying insects) were
recorded on 12 occasions during all months
except November and December. Insects (mostly
Lepidoptera: Geometridae, and Diptera: Tipulidae and Trichoceridae) were also observed on 61
occasions during all months.
Autumn Roost Activities
In September 2001, we captured 8 bats: 3
adult (2 M, 1 F) and 4 juvenile (2 M, 2F) Little
Brown Myotis, and 1 juvenile male Keen’s
Myotis at Gandll K’in Gwaayaay and Huxley
Island. The adult female, at 7.5 g, appeared to be
the only bat that was accumulating fat reserves.
The 2 adult males weighed 5.7 and 5.9 g, while
the juveniles weighed 3.7 to 4.8 g. The 3 adult
Little Brown Myotis were the only bats we
radio-tagged. By following these individuals,
we located 4 day roosts, all in trees. The first
day roost was located on Huxley Island in a
Western Hemlock (Tsuga heterophylla) snag
directly above a 30- to 40-m-high cliff, about
150 m inland from the shoreline. The forest was
second growth Western Hemlock and Sitka
Spruce (Picea sitchensis), with only a few snags
around the cliff. The second roost was in a tree
near the east end of Ramsay Island (52.5776N,
TABLE 4. Summary of acoustic monitoring of bat activities near Sandspit, British Columbia, during the
autumn and winter months of 2000–2001 to 2012–2013.
Month
Number of
nights of
monitoring
September
October
November
December
January
February
March
April
3
22
20
12
20
26
18
39
Total hours of
monitoring
Number of
nights bat
calls were
recorded
Mean (± SD)
number of
calls per hour
Number of
nights feeding
buzzes were
recorded
Mean (± SD)
number of
feeding buzzes
per hour
5.7
99.2
111.1
53.2
81.5
88.2
62.1
178.8
3
11
6
0
7
11
14
17
81.9 ± 70.9
7.8 ± 31.9
0.2 ± 0.4
0
4.9 ± 18.1
1.9 ± 5.4
2.0 ± 3.0
1.5 ± 3.3
2
2
0
0
3
3
3
1
12.3 ± 16.6
15.4 ± 71.9
0
0
0.6 ± 2.7
0 ± 0.1
0.1 ± 0.3
0 ± 0.1
296
NORTHWESTERN NATURALIST
131.3816W). The tree, a large dead Western
Redcedar (Thuja plicata) with no bark, stood on a
ridge (approximately 100 m asl). The tree was
not obvious from offshore. The third roost was
in a dead Western Redcedar snag (60–70 cm
dbh) on Huxley Island, about 15 m from the
shoreline. There was no bark, but a crack or
crease ran up the length of the trunk, and the
signal from the roosting bat seemed to come
from fairly high up. At least 2 other snags were
nearby. The fourth roost was in a large (1.5 m
dbh; 30 m tall), living Western Redcedar about
30 m inland from the third roost. The 3 day
roosts found on Huxley Island were within 300
to 400 m of the capture location, whereas the
day roost on Ramsay Island was approximately
4.2 km from the capture site. The third bat that
we radio-tagged was not relocated after the
night it was released. We also checked a cave
and a mine adit on Huxley Island but found
little evidence of use by bats. During September
2001, the effect of wind and rain on bat emergence and activity at Gandll K’in Gwaayaay was
evident, with little bat activity on nights of heavy
rain.
DISCUSSION
Both the results of our monitoring of potential
hibernation sites and the temperature data
obtained from the Sandspit airport support
our hypothesis that it would be feasible for bats
to hibernate in trees on Haida Gwaii. Specifically, temperatures we recorded in trees were
well within the range of optimal hibernaculum
temperatures predicted by Humphries and
others (2002); they were also similar to those
documented in bat hibernacula on Vancouver
Island (Mather and others 2000).
The results of our calculations of energy
requirements also support our hypothesis that
a bat could hibernate inside our study trees.
Specifically, our estimates of fat reserves required for a bat to survive a winter were well
within the limits of the 1.5 to 3.5 g of fat reserve
predicted to be needed by Humphries and
others (2002). These estimates amount to 29.3
to 32.7% of pre-hibernation body mass, which,
while higher than the 25% reported by Fenton
(1970), is remarkably similar to Kunz and others
(1998) estimates of 29.6 to 32.9% mass gain in
autumn for Little Brown Myotis in Vermont. In
a similar species, Johnson and others (1998)
95(3)
found that Indiana Myotis (Myotis sodalis)
experienced overwinter mass losses of up to
32.8%.
We did record periods when bats would have
to compensate for below freezing temperatures
in 11 of 16 data sets, but most were of short
duration. Although not common, periodic belowfreezing temperatures have been documented in
caves where Big Brown Bats (Eptesicus fuscus),
Silver-haired Bats, and Townsend’s Big-eared
Bats (Corynorhinus townsendii), as well as Little
Brown Myotis, Northern Long-eared Myotis, and
Eastern Small-footed Myotis (Myotis leibii) hibernated (Brack and Twente 1985; Webb and others
1996; Best and Jennings 1997). A recent study in
Alberta also revealed that Big Brown Bats were
active at temperatures as low as 286C (Lausen
and Barclay 2006), so wintering bats are not
averse to below-freezing temperatures.
Perhaps a greater concern for hibernating bats
is that temperatures periodically become too
warm, as warm temperatures generally result in
higher metabolic rates (although see Speakman
and others 1991; Dunbar and Brigham 2010).
This is evident in our calculations of fat
requirements, in that our 2 highest estimates
were for winter 2002–2003, the warmest in our
study. Although not taken into account in our
model, warm temperatures would also likely
result in more frequent arousals (Twente and
others 1985; Dunbar and Brigham 2010), which
further increase the amount of fat required to
survive winter. One trade-off of warm temperatures would be that insects may become active,
thereby creating foraging opportunities for bats.
Another is that energy could be saved if
arousals are timed to coincide with warming
temperatures.
Another concern for a bat hibernating in our
study trees would be the greater temperature
fluctuations it would experience as compared to a
subterranean hibernaculum. While Davis and
others (2000) reported stable temperatures with
variances of only 0.02 to 0.246C at hibernacula on
Vancouver Island, we calculated variances generally of 2.5 to 4.46C for temperatures inside our
study trees, and 6.1 to 7.66C under the bark. These
fluctuations could also result in more frequent
arousals, which would represent an added
energetic cost for a bat hibernating in a tree.
Our finding of California Myotis in the
vicinity of Sandspit during all winters that we
WINTER 2014
BURLES AND OTHERS: WINTER BAT ECOLOGY ON HAIDA GWAII
monitored provides evidence that this species
was not hibernating in caves or mines given that
the nearest underground mine and the nearest
known cave are approximately 100 and .40 km
away, respectively. California Myotis were also
the only species for which a winter specimen
was collected. Thus, these bats were likely
roosting in nearby trees or buildings. This is
not surprising, as they have previously been
identified as being active during winter elsewhere
in British Columbia (Nagorsen and others 1993)
and western Washington (Falxa 2007).
Little Brown Myotis were relatively common
throughout summer and into autumn, but they
were not acoustically detected at all between
late October and late April. This result is similar
to what has been found in southeastern Alaska,
where radio-tagged Little Brown Myotis disappeared in late October (K Blejwas, Alaska
Department of Fish and Game, pers. comm.),
possibly suggesting that they migrate somewhere on or off the islands to hibernate.
Our efforts to track radio-tagged Little Brown
Myotis to their hibernacula failed, in large part
because we started too early in the season. We
assumed that by early September they would be
migrating towards their hibernaculum, where
they would swarm and mate before entering
hibernation. What we found was that by late
September radio-tagged bats had not moved
any distance from where they were captured
and were still using trees as roosts. We also
found that only 1 of the 3 adults that we
captured had begun to accumulate fat reserves.
None of the 5 juveniles captured had accumulated any fat reserves, but juveniles generally
begin this process much later than adults (Kunz
and others 1998). Because the weather was
deteriorating, and foraging opportunities were
becoming rarer, we expected that bats would be
in the latter stages of preparation for hibernation by late September. Our investigation of
potential underground hibernacula at this time
also failed to find any bats.
The almost complete absence of winter
records of Keen’s Myotis is not unexpected. It
is not a common bat on Haida Gwaii, and its
low intensity echolocation call is not easily
recorded. The lack of winter records of the
Silver-haired Bat in winter is surprising, as it is
one of the most common bats to be active in
winter elsewhere (Nagorsen and others 1993;
297
Falxa 2007). Of the 4 species found on Haida
Gwaii, this is the only species that is likely to be
capable of migrating off the islands.
The fact that insects were active during
winter indicates that there were likely opportunities for bats to forage and replenish their fat
reserves during periodic arousals. Although
there did not appear to be any clear correlation
between insect presence and bat activity, foraging ‘‘buzzes’’ were recorded on 5 nights when
insects were also observed. Stomach contents of
the single California Myotis collected in winter
included 1 winter cranefly (Diptera: Trichoceridae), evidence that it had been foraging. While
active foraging would increase fat depletion, it
could also allow for further fat accumulation if
insects were abundant.
While our study has demonstrated that it is
feasible for bats to hibernate in trees on Haida
Gwaii, our efforts to determine where they were
actually hibernating were less successful. Our
acoustic evidence suggests that California Myotis are wintering either in trees or in buildings
around Sandspit. The other 3 species found on
Haida Gwaii seem to disappear in autumn,
however, suggesting that they may be migrating
to a hibernaculum. Given the serious negative
impact that white-nose syndrome is having on
eastern populations of hibernating bats, determining where the bats of Haida Gwaii overwinter is a critical component in planning for
their protection. We recommend that all known
caves and mines be investigated in winter for
the presence of bats, and that further radio
telemetry studies be carried out to determine
where bats are roosting in late autumn.
ACKNOWLEDGMENTS
We thank World Wildlife Fund (Canada) and
Environment Canada for their support through the
Endangered Species Recovery Fund, and Gwaii
Haanas National Park Reserve and Haida Heritage
Site for providing logistical support for our research.
The work was also supported by Research and
Equipment Grants from the Natural Sciences and
Engineering Research Council (Canada) to RMRB,
RMB and MBF. We thank T Jung, K Blejwas and an
anonymous reviewer for comments on a previous
draft of this manuscript.
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Submitted 10 October 2012, accepted 10 November
2013. Corresponding Editor: Thomas S Jung.