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Roost environments for bats using abandoned mines in southwestern Montana : a preliminary assessment (Volume 2001) online

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ciliolabrum we captured at the portal of the
Unnamed Adit #1 on 11 June were non-lactating
females (teats visible, however). The two M.
evotis we captured in August at the Unnamed Adit
#2 were lactating females, the only reproductive
female bats we captured. All the other individuals
that we handled were males.

We were unable to fully survey the three largest
mines, McDonald Adit # 1 , Hendricks Mine,
Union Mine, although we investigated 60-70% of
the workings in each. Therefore, it is possible,
even probable, that we missed seeing some bats
during summer in the McDonald Adit # 1 , and in

Table 3. Bats observed during 1998-1999 at abandoned mines in southwestern Montana.
An asterisk following a mine name indicates bats were captured at the mine portal.


Bat species 3

McDonald Adit #1 (shallow)

8 Corynorhinus townsendii (7 Dec), 1 Myotis species (13 Jul)


1 C. townsendii (13 Jul)

McDonald Adit #2

25 C. townsendii (13 Jul)

Gypsum Adit #2

7 C. townsendii (6 Jan), 1 Myotis species (6 Jan)

Gypsum Adit #1

1 C. townsendii (6 Jan)

Unnamed Adit #2*

5 M. ciliolabrum (6 Aug, 17 Aug), 1 M. evotis (6 Aug, 17 Aug)

Unnamed Adit #3*

2 M. ciliolabrum (7 Aug)

Unnamed Adit #1 *

1 C. townsendii (11 Jun), 8 M. ciliolabrum (11 Jun)


1 C. townsendii (11 Jul), 2 M. ciliolabrum (1 1 Jul)

Hendricks First Drift

None (scattered droppings)

Graeter Tunnel

None (scattered droppings)

Main Drift

1 M. ciliolabrum (4 Dec), 1 Eptesicus fuscus (4 Dec)

Solution Cavity

None (concentrated droppings)

West Drift

1 M. ciliolabrum (13 Jun)

Ruth & Copper Bottom

None (few droppings)

Townsend's Big-eared Bat Corynorhinus townsendii. Western Small-footed Myotis
Myotis ciliolabrum, Western Long-eared Myotis Myotis evotis. Big Brown Bat Eptesicus

the Hendricks Mine during summer and especially
in winter. Low netting success at the portal of the
Union Mine, coupled with our internal survey,
suggests to us that this mine is unlikely to support
relatively large numbers of bats even in areas we
did not reach.

Bats were present during winter at locations with
mean winter temperatures of 4.4-9. 1 °C and mean
relative humidity between 74-84% (Table 2).
Mine sites where we observed bats during the day
in summer (either maternity or day roosts) were
the warmest (1 0.5- 12.2 °C) among the data
logger locations (t = 4.89, P < 0.00 1 ; adjusted for
unequal variances). However, occupied sites in
summer were not necessarily the most humid.
Microclimate conditions at C. townsendii roosts
(Tables 2 and 3, Appendix 1) were cold during
winter (averages of 4.4 and 7.5 °C), but relatively
warm during summer (1 1 .3 and 1 1 .9 °C). Rela-
tive humidity at C. townsendii roosts averaged
74.0 and 83 .4% in winter, 9 1 .7 and 1 00% in


Roost environments: Abandoned mines provide
suitable environments for a variety of roosting
purposes for bats (Pierson et al. 1 99 1 , Tuttle and
Taylor 1 994, Berts 1 997, Sherwin et al. 2000).
Abandoned mines in northwestern North America
are often used as hibernacula and day or night
roosts rather than maternity roosts because mine
temperatures are too cold and energy-expensive
for norma! rates of development of young bats
(Dwyer 1971). The results of our study in south-
western Montana of mine features and microcli-
mates favored by bats, particularly C. townsendii,
conform to general patterns for western North
America. Our study was hampered by lack of
visits to each mine during the four seasons to
determine with certainty the seasonal use of each
by bats. Nevertheless, we documented the long-
term climate of several abandoned mines over an

elevation gradient, and several preliminary conclu-
sions regarding roost use by bats in this portion of
Montana are possible.

We found only one mine (McDonald Adit #2)
used as a maternity roost, by C. townsendii, and
it was at the lowest elevation of the mines studied
(Table 1). Mean June- July temperature near this
colony was about 12 °C (Table 2), which is much
colder than at maternity sites in California (Pierson
etal. 1991). It is possible the McDonald Adit #2
maternity roost moved after our July visit to
warmer temperatures nearer the mine portal.
Similar behavior has been documented for Califor-
nia maternity aggregations after young are bom in
late July and early August (Pierson et al. 1 99 1 ).
We did not get close enough to the McDonald
colony to determine if young bats were present
when we retrieved our data logger on 1 3 July.
There are few temperature and relative humidity
data for other C. townsendii maternity roosts in
Montana. Temperature was 1 8 °C beneath a
maternity roost of about 75 C. townsendii in a
ceiling dome of Toeckes Cave (1 524 m elevation)
on 23 August 1 999 (S. Martinez personal commu-
nication). Temperature was likely at least a few
degrees warmer closer to the roost.

Summer bat use of mines declined with increased
elevation in southwestern Montana (Hendricks et
al. 1999). The most plausible explanation for this
pattern is that mean mine temperature declined
significantly as elevation increased (Figure 2),
making higher elevation mines less attractive to
bats for roosting. This is especially true for female
bats (Cryan et al. 2000) because of increased
energy demands related to reproduction. Bats
found at high elevations in western North America
tend to be males or non-reproductive females
(Storz and Williams 1 996, Cryan et al. 2000).
Currently, little is known about the upper elevation
limit for caves and mines used by bats in Montana.
Little Ice Cave (2493 m elevation) is the highest
known hibernation roost in the state (Madson and
Hanson 1 992). There is also considerable activity
by several species of Myotis at the mouth of this

cave in summer, although cave temperature
throughout is 3 .3 °C (Worthington 1 99 1 ) making it
too cold for use as a maternity roost.

As our data across a range of elevations show
(Table 2, Appendix 1 ), mines in western Montana
generally provide relatively cold roost environ-
ments for bats regardless of season. Greatest use
of abandoned mines by bats in western Montana
is for day/night roosts and hibernacula. Many
abandoned mines in southwestern Montana
present bats with a variety of summer microcli-
mates (Table 2) and are used briefly as night
roosts (Hendricks et al. 1 999), where meals are
digested in relative safety. However, hibernacula
are the best-documented roost climates in Mon-
tana, although data are usually point (single date)
samples, and bat species found hibernating often
are unidentified to species. Fortunately, the
exception is C. townsendii, because it is relatively
easy to identify, even when torpid and undis-

In Montana, C. townsendii use caves and mines
across a broad range of elevations for hibernation
roosts (Table 4). Torpid C. townsendii have

been found from November through April in sites
where the respective ranges of temperature and
relative humidity are-1 .0-8.0 °C and 50-100%
(see also Table 2). Number of hibernating indi-
viduals at each of these sites (Table 4) was < 20,
although larger winter numbers have been re-
ported in appropriate winter roosts in the lower-
elevation plains of eastern Montana (Swenson
1 970), where few surveys have been conducted.
The data presented in Table 4 suggest that roosts
below 2000 m elevation may routinely support
larger winter aggregations of C. townsendii. This
pattern could arise because arid landscapes often
favored by this species (Sherwin et al. 2000) are
found at lower elevations in the region, or because
maternity roosts are often < 20 km from hiber-
nacula (Humphrey and Kunz 1 976, Kunz and
Martin 1982, Dobkin et al. 1995) and are prob-
ably more abundant at lower elevations. Microcli-
mates for Montana hibernacula of C townsendii
are similar to those reported in the literature from a
number of western and midwestern states
(Pearson et al. 1 952, Twente 1 955, Twente 1 960,
Humphrey and Kunz 1976, Genter 1986, Pierson
et al. 1991, Webb et al. 1 996, Choate and
Anderson 1 997, Kuenzi et al. 1 999), with winter

Table 4. Summary of point-sample (single date) microclimate data for Townsend's Big-
eared Bat {Corynorhinus townsendii) hibernacula in Montana. Temperature (T) and
relative humidity (RH) data were recorded near hibernating bats using a sling


Elev (m)


No. bats


RH (%)


McDonald Adit #1


7 Dec





Azure Cave


12 Nov





Gypsum Adit #2


6 Jan





Gypsum Adit #1


6 Jan





Tate-Poetter Cave


19 Apr





Toeckes Cave


12 Feb





Four-eared Bat Cave


26 Feb





Frogg's Fault Cave


28 Feb





Dandy Mine


4 Mar





Lisbon Mine


4 Mar





Big Ice Cave


18 Mar





Mystery Cave


20 Mar





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data, 2000; 5) Madson and Hanson 1992.


roost temperature typically ranging between -1 .5-
10.0 °C. However, in some California locations
roost temperature near torpid individuals may
reach 2 1 .0-25.0 °C (Pierson et al. 1991, Webb et
al. 1 996), much warmer than for any Montana

Management Implications: There are two
major approaches for assessing abandoned mines
for bats: external and internal surveys (Altenbach
1 995, Navo 1 995). During external surveys data
are gathered on the number and dimension of all
entrances (portals), airflow, outside air tempera-
ture, presence of standing water, and visual sign of
bats (carcasses, roosting bats, droppings); one
portal survey in spring, one in summer, and two in
fall are recommended (Navo 1 995). Use of
electronic bat detectors can aid in portal surveys.
Internal surveys allow direct measurement of mine
temperature and relative humidity, and also an
assessment of the extent of underground workings
and their configuration as well as evidence of bats
at specific locations within the mine. Cold season
internal surveys can determine both summer and
winter use, whereas warm season surveys can
determine only summer use.

Our analysis identified few mine characteristics
measurable from external surveys that are good
predictors of mine suitability for bats, with the
exception of obstructions across portals that inhibit
or preclude bat access (Hendricks et al. 1 999).
Mine temperature is an important feature for roost
selection by bats (Dwyer 1 97 1 , Humphrey 1975),
and relative humidity may also be important (Betts
1 997). We found a significant negative relation-
ship between elevation and summer or winter mine
temperature (Figure 2), but not between elevation
and relative humidity; mines at higher elevation
were colder year round, but not necessarily less
humid. Mean mine temperature during both
summer and winter was highly correlated (Figure
3), indicating that temperature taken during one
season is a good predictor of temperature during
the other season in the same mine; this pattern was
also found for relative humidity. Nevertheless,
obtaining these measurements required going

underground. Furthermore, we found consider-
able short-term variation in temperature and/or
relative humidity in most of the mines we moni-
tored (Appendix 1), making questionable the
characterization of their year-round climate from
data obtained during a single visit (Sherwin et al.
2000). If surveys are restricted to one or two
visits because of monetary or logistical limitations,
the potential for significant short-term variation
should be kept in mind when characterizing the
mine climates.

We also found that mines with climates largely
unsuitable for use by bats may contain areas within
them that can be and are used (Table 2, Appendix
1 ). It is not possible to identify these internal
microsites from external surveys, with the possible
exception of the shallowest mines with workings
completely visible from the portal. Identification of
hibernacula, the most likely mine roosts to be used
over several continuous months in Montana, is
impossible from external survey alone. Further-
more, internal survey is the quickest and least
labor/time intensive method for determining mine
suitability for bats in all seasons (Pierson et al
1 999). We therefor suggest that, where safe,
internal survey is the preferred method for assess-
ing mine use and suitability for bats. Where mine
entry is impossible or unsafe, external survey at the
portal must suffice. In these cases it is critical that
surveys are conducted at the appropriate time.
Possible hibernation activity is detected best in fall
(September and October) when bats swarm at
their hibernation roosts. Maternity use of mines is
detected best in summer (July and August) when
females are pregnant or lactating.

We recommend that all abandoned mines sched-
uled for reclamation on public lands receive
proper evaluation as bat habitat prior to closure,
whether by external or internal survey. Protocols
for mine evaluation are presented in the conserva-
tion assessment and strategy for the Townsend's
Big-eared Bat, C. townsendii (Pierson et al.
1 999), as well as Altenbach (1 995) and Navo
(1 995), and are appropriate for all mine-dwelling
bat species in Montana.



Altenbach, J. S. 1995. Entering mines to survey
bats effectively and safely. Pp. 57-61 In
Inactive mines as bat habitat: guidelines for
research, survey, monitoring and mine
management in Nevada (B. R. Riddle,
ed.). Biological Resources Research
Center, University of Nevada, Reno, NV.

Betts, B. J. 1997. Microclimate in Hell's Canyon
mines used by maternity colonies of
Myotis yumanensis. Journal of Mammal-
ogy 78:1240-1250.

Choate, J. R., and J. M. Anderson 1 997. Bats of
Jewel Cave National Monument, South
Dakota. Prairie Naturalist 29:39-47.

Cryan, P. M., M. A. Bogan, and J. S. Altenbach.
2000. Effect of elevation on distribution
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Dakota. Journal of Mammalogy 81:719-


Dobkin, D. S., R. D. Gettinger, and M. G.

Gerdes. 1995. Springtime movements,
roost use, and foraging activity of
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perspective. Mammalia 35:424-455.

Hendricks, P., D. L. Genter, and S. Martinez.

2000. Bats of Azure Cave and the Little
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Field-Naturalist 114:89-97.

Hendricks, P., D. Kampwerth, and M. Brown.
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for bat use on Bureau of Land Manage-
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1997-1998. Unpublished report, Mon-
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MT. 29 p.

Humphrey, S. R. 1975. Nursery roosts and
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Kuenzi, A. J., G. T. Downard, and M. L.

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Appendix 1. Continuous temperature (solid line)
and relative humidity (broken line) profiles for
1 998- 1 999 from nine mines in southwestern
Montana (see Table 1 for additional details).
Note that scales vary from figure to figure and that
time periods of continuous recordings also vary.
Townsend's Big-eared Bat {Corynorhinus
townsendii) was documented underground at the
first four locations (McDonald Adit #2, McDonald
Adit #1 both sites, Gypsum Adit #2) and captured
in summer at the portals of the next two locations
(Unnamed Adit #1, Union Mine). McDonaldAdit
#2 was a maternity site.


McDonald Adit #2


12 -


? 10-



E 9-






McDonald Adit #1 (Shallow)

14 -

12 -

10 •
0) 8 •


McDonald Adit #1 (Deep)




Gypsum Adit #2




7 -]




' 1 -




Feb Mar Apr May Jun Jul


■ 100

• 90

-70 2





Unnamed Adit #1




Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul

Aug Sep


Unnamed Adit #2


Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep


Unnamed Adit #3


14 -





9 -








1 -



Ruth & Copper Bottom

Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep


Hendricks Graeter Tunnel

Hendricks First Drift

Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep

Hendricks Main Adit

Hendricks Solution Cavity

Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep




- 70

- 50

Oct Nov Dec Jan Feb Mar Apr May Jun Ju! Aug Sep

Hendricks West Drift

14 -
13 -
12 •

8 ■

7 -
6 •
5 -
4 -
3 -
2 -
1 -

TT"^ u ■■


Vtf . „ A"

II ^





Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep



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